Add ability to filter patchset comments.

Zuul layouts can now specify that jobs get run when comments match
a filter. Trigger layouts would look like:
  trigger:
    - event: comment-added
      comment_filter:
        - reverify
        - recheck

This would trigger a job whenever comments containing the string
"reverify" or "recheck" are added to a change.

Change-Id: I3dd75abbf75686b3929dd2bb412b02740911d6ee
4 files changed