Install mechanism packages for ml2 on RedHat

When using linuxbridge and/or openvswitch as ml2 mechanism_drivers,
install the necessary linuxbridge/openvswitch plugin package(s).

Change-Id: Id5bb4ab96701e0da15c82557f07ed304db76e65a
Closes-Bug: #1269672
Merge "Install mechanism packages for ml2 on RedHat"