flat_networks - Don't hardcode public network name
flat_networks - Don't hardcode public network name

Use the PUBLIC_NETWORK_NAME variable instead of hardcoding
it when setting the [ml2_type_flat]/flat_networks option.

Change-Id: I8bfc37089ec90eb06ee41d85744dad0f3f734c16

openstack-dev/devstack,nawawi/openstack,dtroyer/devstack,dtroyer/devstack,openstack-dev/devstack,nawawi/openstack