added support for creating network devices for lxc

since libvirt does not support passing devices to lxc (although it is
possible with plain lxc), the driver now creates a bridge, passes this
to the lxc and registering it to contrail.

Requires https://review.opencontrail.org/#/c/2806/

Change-Id: I9c427f420831359d45c8963e82e580ad298bc3ce