Default to admin viewable mailman membership

Change-Id: I0734d37ce65d817208c3b8a37e9b322d235ae8e1
Fixes: bug #1021493
Reviewed-on: https://review.openstack.org/28224
Reviewed-by: Rainya Mosher <rainya.mosher@rackspace.com>
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Reviewed-by: Stefano Maffulli <stefano@openstack.org>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins