Definition of network driver interface. Also removed
the floating_ip attributes of VIP because they are not
needed at this point. Also renamed net_port_id to just
port_id and subnet_id to network_id just to be a little
bit more generically clear.
Change-Id: Ic82cb2ab25fbba7dc8caa875552f4caeafb0e4af
Implements: bp/network-driver-interface