Bind HOST_IP to PublicIP instead of localhost to ensure proper endpoint registration

* Devstack registers endpoints in keystone to host ip, instead of localhost
* We also need to 'export' the SERVICE_HOST settings, so test.conf is set up correctly
* using a simpler command as suggested by hub_cap to extract default iface

fixes bug: 1115352

Change-Id: I6aa7440c06bc4da595f3abf8ac50163d95af76d1
Merge "Bind HOST_IP to PublicIP instead of localhost to ensure proper endpoint registration"