Begin is_service_enabled() cleanup

This converts the special cases in the is_service_enabled() function to call
individual functions declared by the projects.  This allows projects that
are not in the DevStack repo and called via the extras.d plugin to handle
an equivalent service alias.

* Ceilometer
* Cinder
* Glance
* Neutron
* Nova
* Swift

Change-Id: I5b14226b7875e15ad1e375b532adbd257fbe4296
TODO: remove the tests from is_service_enabled() after a transition period
Merge branch 'trove-plugin' into dt-test
Merge branch 'code-cleanup' into dt-test
Merge branch 'is-service-enabled' into dt-test