give tempest-core approve permission on stable branch

because tempest is under openstack stable branch +2 actually defaults
to stable-maint, not tempest-core. We should have +2 to be able to
approve backports to tempest.

Change-Id: Ie610d828ab04054941010f68d917fee27aca6ca1
Reviewed-on: https://review.openstack.org/28167
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins