Explicitly set "host" address for VNCProxy

Override default settings and explicitly bind to the management address.
Work around strange interpretation of bridge_uplinks

Explicitly create the external bridge (br-ex) on the node running
the L3 agent.

The neutron module creates bridge_uplinks only when bridge_mappings is
non-empty. That's bogus for GRE configurations, so create the "bridge
uplink" manually.