openapproved: ignore patches with a negative VRIF

In addition to ignoring patches with a negative review, ignore patches
that fail jenkins tests. As a negative VRIF implies the patch isn't
ready to be approved again.

Clarify docstring, to point trying to identify trivial rebases only.

Change-Id: I812ad787cf6aa06d1ea05b2a0b70574b907b8777
Reviewed-on: https://review.openstack.org/36416
Approved: Russell Bryant <rbryant@redhat.com>
Reviewed-by: Russell Bryant <rbryant@redhat.com>
Tested-by: Jenkins