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
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