[openstack|network] Add support for OpenStack Networking LBaaS extension

Add support for OpenStack Networking [Load Balancer as a Service (LBaaS)](http://docs.openstack.org/api/openstack-network/2.0/content/lbaas_ext.html) extension:
* Load balancing pools
* Load balancing members
* Load balancing health monitors
* Load balancing VIPs