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

doc/source/index.rst

@@ -45,6 +45,7 @@ Development Documents
    :maxdepth: 1
 
    devref/index
+   devref/common
 
 Man Pages
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