Clean up Glance service checks

Use $GLANCE_SERVICE_NAMES in all is_service_enabled calls where
one or more services are checked; i.e. replace 'glance' as the argument
to eliminate the hack in the is_service_enabled() function.

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

Change-Id: I7f1b1b20b2f7f525e4eb820d4e25af603e45bd12
Merge branch 'info-update' into dt-test
Merge branch 'enabled-glance' into dt-test
Merge branch 'plugin-functions' into dt-test
Merge branch 'samples-update' into dt-test
Merge branch 'cleaner' into dt-test