Unify Manager._update behaviour

Now _update call usually returns an instance of self.resource_class.
This simplifies the code and makes novaclient closer to keystoneclient.

Also, update hosts and services API according to changes on nova.
(If50a6b6e20f9b3fe66d486bb9b15d3eb4b62daf9).

Change-Id: I447e49e5fce0afba8a9c1a5df6dfa7200cc93e18