gerrit: Add support for on changed restored/abandoned.

These triggers are supported starting at gerrit-trigger 2.8.0.

Using thoses triggers in a previous version should not cause any
problem.

Change-Id: I388cfb9424dd16dc436010c492e59dd0e896b69a
Reviewed-on: https://review.openstack.org/24264
Reviewed-by: James E. Blair <corvus@inaugust.com>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Anita Kuno <akuno@lavabit.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins

samples/gerrit-trigger.yaml

@@ -2,7 +2,9 @@
     name: test-gerrit-trigger
     triggers:
         - gerrit:
+            triggerOnChangeAbandonedEvent: true
             triggerOnChangeMergedEvent: true
+            triggerOnChangeRestoredEvent: true
             triggerOnCommentAddedEvent: true
             triggerOnDraftPublishedEvent: true
             triggerOnPatchsetUploadedEvent: true