provider/aws: spot_instance_request

This is an iteration on the great work done by @dalehamel in PRs #2095
and #2109.

The core team went back and forth on how to best model Spot Instance
Requests, requesting and then rejecting a separate-resource
implementation in #2109.

After more internal discussion, we landed once again on a separate
resource to model Spot Instance Requests. Out of respect for
@dalehamel's already-significant donated time, with this I'm attempting
to pick up the work to take this across the finish line.

Important architectural decisions represented here:

 * Spot Instance Requests are always of type "persistent", to properly
   match Terraform's declarative model.
 * The spot_instance_request resource exports several attributes that
   are expected to be constantly changing as the spot market changes:
   spot_bid_status, spot_request_state, and instance_id. Creating
   additional resource dependencies based on these attributes is not
   recommended, as Terraform diffs will be continually generated to keep
   up with the live changes.
 * When a Spot Instance Request is deleted/canceled, an attempt is made
   to terminate the last-known attached spot instance. Race conditions
   dictate that this attempt cannot guarantee that the associated spot
   instance is terminated immediately.

Implementation notes:

 * This version of aws_spot_instance_request borrows a lot of common
   code from aws_instance.
 * In order to facilitate borrowing, we introduce `awsInstanceOpts`, an
   internal representation of instance details that's meant to be shared
   between resources. The goal here would be to refactor ASG Launch
   Configurations to use the same struct.
 * The new aws_spot_instance_request acc. test is passing.
 * All aws_instance acc. tests remain passing.

builtin/providers/aws/provider.go

@@ -128,6 +128,7 @@ func Provider() terraform.ResourceProvider {
 			"aws_s3_bucket":                    resourceAwsS3Bucket(),
 			"aws_security_group":               resourceAwsSecurityGroup(),
 			"aws_security_group_rule":          resourceAwsSecurityGroupRule(),
+			"aws_spot_instance_request":        resourceAwsSpotInstanceRequest(),
 			"aws_sqs_queue":                    resourceAwsSqsQueue(),
 			"aws_sns_topic":                    resourceAwsSnsTopic(),
 			"aws_sns_topic_subscription":       resourceAwsSnsTopicSubscription(),