Add --no-build switch for invoke local to use already compiled output

[HyperBrain]

What did you implement:

Closes #341
Closes #275

How did you implement it:

On invoke local we check for a --no-build command line switch now and skip the compile step. This enables the requested feature, so that on your CI server where tests are run, you just do a single sls webpack and multiple invoke local --no-build to fire mutliple integration tests onto one build output.
However, enabling local invoke mode --watch together with the switch is not recommended and will NOT disable the compile (somehow that's implicit).

The operation with the new switch will be aborted if the plugin does not find a webpack output directory (i.e. no compile was done before). Additionally a message will be logged as soon as the switch is used.

Off-Topic: Fixed an issue with event spawn for the step offline plugin in this PR.

How can we verify it:

Use the following steps and check that a compile is not triggered by invoke local:

sls webpack
sls invoke local --no-build --function=XXXXX
sls invoke local --no-build --function=YYYYY

Todos:

• Write tests
• Write documentation
• Fix linting errors
• Make sure code coverage hasn't dropped
• Provide verification config / commands / resources
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: YES
Is it a breaking change?: NO