Fix periodic job branch/labels.

The ZUUL_BRANCH parameter should be of type 'string', not
a node label.

Change-Id: I30e6c6965e945217ccdfaa1aa19c23f3640ca903
Reviewed-on: https://review.openstack.org/19765
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins