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
Merge branch 'cleaner' into dt-test
Merge branch 'code-cleanup' into dt-test
Merge branch 'enabled-base' into dt-test