Add CodeReview vote ability to the Gerrit trigger

- Made all four of the
  gerrit-build-(successful|failure)-(verified|codereview)-value
  parameters optional, so you can override only the ones you want.

- Also allows for vote values like '+1' in the yaml by first coercing
  to int and then str.

Change-Id: Id2ad7e4a4cf53bd33b3aca80fd95ed1d4b3ff266
Merge "Add CodeReview vote ability to the Gerrit trigger"