Allow a charm to use the new register_get_charm_instance decorator
to register the method to be used when getting a charm instance.
Currently get_charm_instance expects standard OpenStack
versioning, this change allows are a charm to register an
alternative get_charm_instance method that can handle an
alternative versioning system.
Perhaps controversially the default method is not defined in
charms_openstack.charm.defaults. I have kept it in core
because it felt a more natural fit there given it relies on
core._releases but I am open to moving it if that is
preferable.
Change-Id: I42574654bc1f314b49049e80861d4039f8484dff
38de241ce6