Add support for 'recheck no bug'

Change-Id: I01f10fc41f9830671161c65a53d70e7ffd149c08
Reviewed-on: https://review.openstack.org/19868
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins

modules/openstack_project/files/zuul/layout.yaml

@@ -9,7 +9,7 @@ pipelines:
     trigger:
       - event: patchset-created
       - event: comment-added
-        comment_filter: (?i)^\s*recheck( (?:bug|lp)[\s#:]*(\d+))?\s*$
+        comment_filter: (?i)^\s*recheck(( (?:bug|lp)[\s#:]*(\d+))?|( no bug)?)\s*$
     success:
       verified: 1
     failure:
@@ -24,7 +24,7 @@ pipelines:
         approval:
           - approved: 1
       - event: comment-added
-        comment_filter: (?i)^\s*reverify( (?:bug|lp)[\s#:]*(\d+))?\s*$
+        comment_filter: (?i)^\s*reverify(( (?:bug|lp)[\s#:]*(\d+))?|( no bug)?)\s*$
     start:
       verified: 0
     success: