Collect plugin support bits in lib/plugin_functions

* add run_plugin() to perform the loop through the extras.d/*.sh files
* add is_plugin_enabled() to safely test for enabled plugins

Change-Id: I0a330e1a93ed9e8d0e50850908df6072b6500e36
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.

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