CinderStoragePluginCharm should not register self.on_config
as a hook, because the parent class method already will call this
on config changed (OSBaseCharm._on_config).
(also add new tox test targets to keep zuul happy)
The purpose of this new class is to ease development for existing
and upcoming cinder plugins, for instance, the cookiecutter and
solidfire ones. This class makes development much more in line with
the existing reactive libraries.