fix(toolkit): options requiring arguments fail if not supplied

Some options require arguments, but without passing `requiresArg: true`
to `yargs` it will happily accept the flag and put `undefined` or an
empty string in the value (instead of rejecting the invocation).

Add `requiresArg` to a number of options that have this property.

Fixes #2192.

packages/aws-cdk/bin/cdk.ts

@@ -33,7 +33,7 @@ async function parseCommandLineArguments() {
   return yargs
     .env('CDK')
     .usage('Usage: cdk -a <cdk-app> COMMAND')
-    .option('app', { type: 'string', alias: 'a', desc: 'REQUIRED: Command-line for executing your CDK app (e.g. "node bin/my-app.js")' })
+    .option('app', { type: 'string', alias: 'a', desc: 'REQUIRED: Command-line for executing your CDK app (e.g. "node bin/my-app.js")', requiresArg: true })
     .option('context', { type: 'array', alias: 'c', desc: 'Add contextual string parameter.', nargs: 1, requiresArg: 'KEY=VALUE' })
     .option('plugin', { type: 'array', alias: 'p', desc: 'Name or path of a node package that extend the CDK features. Can be specified multiple times', nargs: 1 })
     .option('rename', { type: 'string', desc: 'Rename stack name if different from the one defined in the cloud executable', requiresArg: '[ORIGINAL:]RENAMED' })
@@ -42,20 +42,20 @@ async function parseCommandLineArguments() {
     .option('ignore-errors', { type: 'boolean', default: false, desc: 'Ignores synthesis errors, which will likely produce an invalid output' })
     .option('json', { type: 'boolean', alias: 'j', desc: 'Use JSON output instead of YAML' })
     .option('verbose', { type: 'boolean', alias: 'v', desc: 'Show debug logs' })
-    .option('profile', { type: 'string', desc: 'Use the indicated AWS profile as the default environment' })
-    .option('proxy', { type: 'string', desc: 'Use the indicated proxy. Will read from HTTPS_PROXY environment variable if not specified.' })
+    .option('profile', { type: 'string', desc: 'Use the indicated AWS profile as the default environment', requiresArg: true })
+    .option('proxy', { type: 'string', desc: 'Use the indicated proxy. Will read from HTTPS_PROXY environment variable if not specified.', requiresArg: true })
     .option('ec2creds', { type: 'boolean', alias: 'i', default: undefined, desc: 'Force trying to fetch EC2 instance credentials. Default: guess EC2 instance status.' })
     .option('version-reporting', { type: 'boolean', desc: 'Include the "AWS::CDK::Metadata" resource in synthesized templates (enabled by default)', default: undefined })
     .option('path-metadata', { type: 'boolean', desc: 'Include "aws:cdk:path" CloudFormation metadata for each resource (enabled by default)', default: true })
     .option('asset-metadata', { type: 'boolean', desc: 'Include "aws:asset:*" CloudFormation metadata for resources that user assets (enabled by default)', default: true })
-    .option('role-arn', { type: 'string', alias: 'r', desc: 'ARN of Role to use when invoking CloudFormation', default: undefined })
-    .option('toolkit-stack-name', { type: 'string', desc: 'The name of the CDK toolkit stack' })
+    .option('role-arn', { type: 'string', alias: 'r', desc: 'ARN of Role to use when invoking CloudFormation', default: undefined, requiresArg: true })
+    .option('toolkit-stack-name', { type: 'string', desc: 'The name of the CDK toolkit stack', requiresArg: true })
     .command([ 'list', 'ls' ], 'Lists all stacks in the app', yargs => yargs
       .option('long', { type: 'boolean', default: false, alias: 'l', desc: 'display environment information for each stack' }))
     .command([ 'synthesize [STACKS..]', 'synth [STACKS..]' ], 'Synthesizes and prints the CloudFormation template for this stack', yargs => yargs
       .option('exclusively', { type: 'boolean', alias: 'e', desc: 'only deploy requested stacks, don\'t include dependencies' })
       .option('interactive', { type: 'boolean', alias: 'i', desc: 'interactively watch and show template updates' })
-      .option('output', { type: 'string', alias: 'o', desc: 'write CloudFormation template for requested stacks to the given directory' })
+      .option('output', { type: 'string', alias: 'o', desc: 'write CloudFormation template for requested stacks to the given directory', requiresArg: true })
       .option('numbered', { type: 'boolean', alias: 'n', desc: 'prefix filenames with numbers to indicate deployment ordering' }))
     .command('bootstrap [ENVIRONMENTS..]', 'Deploys the CDK toolkit stack into an AWS environment')
     .command('deploy [STACKS..]', 'Deploys the stack(s) named STACKS into your AWS account', yargs => yargs
@@ -68,8 +68,8 @@ async function parseCommandLineArguments() {
       .option('force', { type: 'boolean', alias: 'f', desc: 'Do not ask for confirmation before destroying the stacks' }))
     .command('diff [STACKS..]', 'Compares the specified stack with the deployed stack or a local template file, and returns with status 1 if any difference is found', yargs => yargs
       .option('exclusively', { type: 'boolean', alias: 'e', desc: 'only diff requested stacks, don\'t include dependencies' })
-      .option('context-lines', { type: 'number', desc: 'number of context lines to include in arbitrary JSON diff rendering', default: 3 })
-      .option('template', { type: 'string', desc: 'the path to the CloudFormation template to compare with' })
+      .option('context-lines', { type: 'number', desc: 'number of context lines to include in arbitrary JSON diff rendering', default: 3, requiresArg: true })
+      .option('template', { type: 'string', desc: 'the path to the CloudFormation template to compare with', requiresArg: true })
       .option('strict', { type: 'boolean', desc: 'do not filter out AWS::CDK::Metadata resources', default: false }))
     .command('metadata [STACK]', 'Returns all metadata associated with this stack')
     .command('init [TEMPLATE]', 'Create a new, empty CDK project from a template. Invoked without TEMPLATE, the app template will be used.', yargs => yargs