Set ovs_bridge in nova during devstack ml2 deployment.
Set ovs_bridge in nova during devstack ml2 deployment.

Change-Id: I2b5c3d44a0cc16c8a6b073c41765b4ae29fb63b8
Closes-Bug:OPENSTACK-1789

naveensan1/nuage-openstack-neutron,nuagenetworks/nuage-openstack-neutron,naveensan1/nuage-openstack-neutron,nuagenetworks/nuage-openstack-neutron