Don't write the "bridge_mappings" setting when using gre tunnels

It's unneeded and breaks the openvswitch agent's automatic creation of the
br-tunnel bridge. This allows us to cleanup the common_agent recipe a little
bit as we don't need to create the ovs-bridge there anymore.
Merge pull request #119 from rhafer/br-tunnel-autocreate

Don't write the "bridge_mappings" setting when using gre tunnels