Make default nova_url use a version

Make default nova_url use a version

The default nova_url for neutron is missing an API
version number. This can cause requests to fail
because the Nova /versions API cannot respond
to Neutron notification requests.

It seems reasonable for the default value to
at least have a chance at being correct so
this patch upgrades the default Nova API url to
use the Nova 'v2' API.

Related-bug: #1298640
Change-Id: Ib1449de84fbc01fb704ebfe4a016ac8f4932be96
(cherry picked from commit c09a14089a5ca7cd3093895ee0248876499a6d06)