Implement nova client plugin

This moves the client creation code out of Clients._nova() into
its own client plugin.

Raising nova exceptions has been moved to
heat.tests.v1_1.fakes.fake_exception since it is actually impossible
to directly create a nova exception object which works on both
novaclient 2.17 and (yet to be released) 2.18.
nova.exceptions.from_response is stable across novaclient releases
so all test exceptions are now raised using from_response.

Change-Id: Id68ff32c381c9c8c06f3712736d1888d1d0e8110