Add floating ip table for neutronv2 datasource driver

If an instance is attached with the port that is associated with floating IP, it is very different with regular instance, the instance can be accessed from internet, so maybe it need be take care of with special security group rules, in these scenario, Congress should fetch the floating IP information from neutron, so we need to add a new floating IP table and related translator for neutronv2 datasource driver.

Although the floating_IPs table exist in nova datasource, its data come from nova API floating IP list, actually we should only use nova floating_IPs table in nova-network model, if we deploy neutron, we should fetch floating IP by neutron API, and the current nova "floating_IPs" table lack some important information, like: tenant_id, port_id, floating_network_id, status and router_id, these can't be got by nova API. On the other hand nova floating ip list API might be deprecated when the migrating is finished from nova-network to neutron.