Add additional network configuration options to external Openstack CCM (#6083) (#6085)

* Add additional network configuration options to external Openstack CCM (#6083)

* Change the default version of external openstack cloud controller image to v1.18.1 since there was an issue in v1.18.0 where some IPs of the private network were ignored

* Change Network section in external-openstack-cloud-config.j2 to Networking

* Add networking customization information in the openstack documentation