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
Allow log_dir to be set to false in order to disable file logging

This commit allows a user to disable logging to a directory (for
example if syslog is enabled) in order not to login twice.

Change-Id: I0d201b8892cdaea86714b39f7716e763a40a2adc
Closes-bug: #1282296
Merge "Install mechanism packages for ml2 on RedHat"