charms_openstack.bus.discover() can now be used to load the openstack
charm modules placed under lib/charm/openstack/. This is particularly
useful when using the generic provide_charm_instance() code e.g.
```
charms_openstack.bus.discover()
with charms_openstack.charm.provide_charm_instance() as ci:
ci.some_charm_instance_method()
```
Change-Id: Ic1dfb58a37b3f283bbc5b31b5ea192527fc8e57d