Fix doc generation for contrib resources

Since we shouldn't require contrib resources to be installed in order to
generate documentation, this adds an exception to the plugin loader so
that it doesn't try to load the setup.py files that can be found inside
each plugin directory inside contrib/

Closes-Bug: 1403897
Change-Id: Id6ece4379599925dd17d22cef401c8d667a0ac51
Merge "Fix doc generation for contrib resources"