Replace ensure-bridge w/ os-net-config.

This updates the init-neutron-ovs script so that it
dynamically writes out an os-net-config config file
and then runs os-net-config to perform the required
network configuration.

Change-Id: I5f51ecb5c64fc9c13eabc16b2b43b5dda1b38d65
Merge "Replace ensure-bridge w/ os-net-config."