Parse linuxbridge plugins using openstack.common.cfg

Parse linuxbridge plugins using openstack.common.cfg

Implements the blueprint use-common-cfg

In order for the linuxbridge plugin to use the rpc code soon to be merged into
openstack-common, we need to parse our configuration using cfg. Here we
make the most simple, backwards compatible change in that direction.

The same is relevant for the openvswitch implementation

Fixes after comments.

Change-Id: I70fc1898a802cb4198f72741453283d114added2