Make gerrit set PATCH_TO_REVIEW status only in bugzilla

As the PATCH_TO_REVIEW status is only available in bugzilla (not in
phabricator), we restrict setting it to gerrit's bugzilla plugin and
thereby prepare for running both the gerrit's bugzilla and gerrit's
phabricator plugin in parallel.

Bug: T169
Change-Id: Ib1cfc0c3435bebbd2a02b097c1ac7c9b24fb5650