documentation fix
documentation fix

updated "message_broker" to "message-broker" in "ubuntu.rst" under
Add cue service to keystone

Change-Id: I7e32c383c129a0ec203cab7f6e27155346adf916
Closes-Bug: #1555954

openstack/cue,stackforge/cue,stackforge/cue,openstack/cue,openstack/cue,stackforge/cue