Fix Ceilometer undercloud endpoint registration when UI not specified

When USE_UNDERCLOUD_UI setting is not defined the Ceilometer elements
are not installed, however the endpoint is still registered which
causes bad behavior in the Horizon UI. Let's make endpoint registration
dependent on that setting as well.

Closes-Bug: #1332321
Change-Id: I88fcd7b58499b682472d4c29d873d1e769169b5d
Merge "Fix Ceilometer undercloud endpoint registration when UI not specified"