As userdata plugin is doing when executing userdata scripts,
do the same for the scripts executed under the localscripts plugin.
* move the `get_plugin_return_value` to a common place
* properly test the method above
* use it with the localscripts plugin and make sure to choose
the most comprehensive plugin status and reboot requirement
Change-Id: Id3aeab079be39e2c4d3d8bf51ef10ef1c7be68e2