Clean up Glance service checks

Add is_glance_enabled() to lib/glance to enable removing the 'glance'
special-case in the is_service_enabled() function itself.

TODO: remove the glance test from is_service_enabled() after a transition period

Change-Id: I7f1b1b20b2f7f525e4eb820d4e25af603e45bd12
Clean up Cinder service checks

Add is_cinder_enabled() to lib/cinder to enable removing the 'cinder'
special-case in the is_service_enabled() function itself.

TODO: remove the cinder test from is_service_enabled() after a transition period

Change-Id: I6079ba253517d5418f8d193a22d390851696b342
Merge branch 'code-cleanup' into dt-test
Merge branch 'enabled-base' into dt-test
Merge branch 'enabled-cinder' into dt-test
Merge branch 'enabled-glance' into dt-test