Provide commands to run via actions

[jmccann]

This is an attempt to address #57. It includes breaking changes. Would appreciate your thoughts @msuterski

Valid actions would include: validate, plan, apply, plan-destroy, destroy

The following would still have validate, plan, apply by default.

pipeline:
  terraform:
    image: jmccann/drone-terraform:latest

The following would "dry run" and validate and plan only:

pipeline:
  terraform:
    image: jmccann/drone-terraform:latest
    actions:
      - validate
      - plan

The following would validate only:

pipeline:
  terraform:
    image: jmccann/drone-terraform:latest
    actions:
      - validate

The order the actions are listed in are the order they are executed.

[msuterski]

I thinks this format looks great. Having access to destroy would save me from filing another PR 👍

I asked my team to look at it, too, and provide feedback if they have any.

Thanks for working on it.

[dthapa]

@msuterski how goes the validation. this would be a great feature to release.