on_success: perform a step when another succeeds

Any step can have on_success tacked onto it, whose value is a second step to execute only if the parent step succeeds.

The step to execute when the parent step succeeds. If the attached step fails, the outer step is considered to have failed.

The following will perform the second task only if the first one succeeds:

plan:
- get: foo
- task: unit
  file: foo/unit.yml
  on_success:
    task: alert
    file: foo/alert.yml

Note that this is semantically equivalent to the following:

plan:
- get: foo
- task: unit
  file: foo/unit.yml
- task: alert
  file: foo/alert.yml

...however it is provided mainly for cases where there is an equivalent on_failure, and having them next to each other is more clear.