Document how to enable barbican in devstack
Document how to enable barbican in devstack

Change-Id: If5373ea6f8d53607afafa1944607203b206f97b2
Closes-Bug: #1504215

openstack/magnum,jay-lau/magnum,ArchiFleKs/magnum,dimtruck/magnum,ArchiFleKs/magnum,dimtruck/magnum,openstack/magnum