Allow merge commits in zuul.

It has a feature branch.

Change-Id: I9141df6974520a15c79fda66f82b8ca25c768327
Reviewed-on: https://review.openstack.org/29254
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins