ovs-brcompatd: First cut at integration with new config db

ovs-brcompatd: First cut at integration with new config db

This is an extremely lightly tested attempt at switching ovs-brcompatd
from using the config file to the new config db.  There are a lot of
shortcomings in this cut, but we need to make progress on the XenServer
integration, so it's going in now.  Expect changes in the near future.