Allow public router external net to use a non-flat provider network.
Allow public router external net to use a non-flat provider network.

Allow users to auto-create a neutron non-flat providernet public network
and use it for external router interfaces.  By default, keep the existing
flat network type behavior.

Change-Id: I64f71b0c9fcac97b9b84b7d30ee61659b2a690f1

noironetworks/devstack,group-policy/devstack,dtroyer/devstack,group-policy/devstack,noironetworks/devstack,nawawi/openstack,dtroyer/devstack,openstack-dev/devstack,nawawi/openstack,openstack-dev/devstack