Add support to reject changes from approvals

Formally "support for negative requirements"

This change adds support to reject a change from entering a pipeline
based off any existing approvals. This is useful to stop queuing any
changes with a negative vote.

Two new options are added:

reject:
  approval:
    - ...

To the pipelines, and:

reject-approval:
  - ...

To the triggers.

The approval format is the same for the "require approval" section.
Reject approvals could be considered a negative of the require section.

Change-Id: I3369920530e0b7439208b8fd43a9e75994860666
9 files changed