Permit specifying instance networks to use.

Provider.networks can now be specified as a list of dicts containing
the same parameters that 'nova boot --nic ....' would take - e.g.
net-id: 'someuuid'.

Change-Id: Ie34c3636f8a2fce6aae0ceb249294eaba6a92d35
Co-Authored-By: Robert Collins <rbtcollins@hp.com>