Allow Neutron to specify OVS bridge name

(This blueprint is submitted as a specless blueprint.)

Nova currently gets the name of the OVS bridge to use from a nova.conf. configuration option. nova/network/neutronv2/api.py contains the following:

     # TODO(berrange) temporary hack until Neutron can pass over the
     # name of the OVS bridge it is configured with
     cfg.StrOpt('ovs_bridge',
                default='br-int',
                help='Name of Integration Bridge used by Open vSwitch'),

This blueprint proposes allowing Neutron to specify this in the VIF details that Neutron already sends to Nova. The patch should be minimally invasive, as it should only affect the 2 places in the neutronv2.api code that currently read this configuration option. It would grab it from VIF details if present, and fall back to the config option otherwise.

This came up as it would help with implementing the Neutron vlan-aware-vms spec with Neutron's OVS agent. https://review.openstack.org/#/c/243786/ Even if not used there, it's still a very reasonable TODO item to knock out, as the choice of OVS bridge is really something Neutron should be specifying anyway, even if it's almost always br-int.