Support "plan only" on master

[kmanning]

• Allow a user to run the master branch, but as 'plan only'
• When enabled, the confirm and apply steps will be skipped on the master branch
• Allow the user to indicate that the pipeline should fail if un-applied changes are found in any environment.
• UseCase:
  • Each week, I want to trigger all my pipelines to look for un-applied changes
  • When triggered, 'plan' should be run across each environment in the pipeline
  • If any environment has un-applied changes, the pipeline should fail-fast
• Dev Notes: Generally speaking, this should be implemented as a Plugin. Build this behavior into https://github.com/manheim/terraform-pipeline/blob/master/docs/ConditionalApplyPlugin.md if it makes sense for the behavior to live here. If it doesn't make sense, build a new Plugin.

[kmanning]

This affects branches other than master. Should that be the case? - Seems fine for now