Fixup network configuration for ext-net

We currently make the external network a single-node gre network but
this is not at all correct for HA environments - we need a provider
network, which means having a bridge mapping, a flat network
specified, and then because we run the same ovs config everywhere we
need br-ex on the hypervisors too. This is entirely reasonable since
DVR will require this as well (and solve lots of scaling issues...).

Change-Id: I8b63ab51e7e20b235430fad8d786d8da005d84a1
Merge "Fixup network configuration for ext-net"