Set the MAC to the primary interface

Adds support for a new 'primary' interface option exposed via the
object model and JSON parsers which can be used to force the
MAC address on a bridge. Only one interface on a given
bridge (or bond) may be set as the primary interface.

Also, update the ifcfg and eni providers so that they use
OVS_EXTRA (or ovs_extra) to pin the mac accordingly.