rabbitmq-plugins enable no longer needs /bin/true
rabbitmq-plugins enable no longer needs /bin/true

It's unclear why this was needed originally but rabbitmq builds fine
without it.

Change-Id: I604af090b727c50e00af4273fd92ee5b6954393a

GalenMa/kolla,nihilifer/kolla,rahulunair/kolla,mrangana/kolla,stackforge/kolla,GalenMa/kolla,stackforge/kolla,openstack/kolla,openstack/kolla,rahulunair/kolla,stackforge/kolla,nihilifer/kolla,mrangana/kolla