Begin is_service_enabled() cleanup

This lays the groundwork to convert 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 equivalen service alias.

Change-Id: I4879ec51287088f471df867b96e0531bd58908e6
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 'cleaner' into dt-test
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