Evaluate the use entry_points to consume lp-builder-config provided by 'plugins'

[freyes]

At the moment the openstack charms define its lp-builder-config in a python package called charmed-openstack-info, it would be nice if charmhub-lp-tools could pick it up when it's installed, this way there won't be need to keep a local git clone of the repo and instruct to charmhub-lp-tool where it is.

This kind of behavior could be achieved with the use of entry_points - https://setuptools.pypa.io/en/latest/userguide/entry_point.html#advertising-behavior .