Add network API test to create/update a port with 2 IP addresses

Test sequence is as follows:
- Create a network with 2 subnets.
- Create a port on the network with single IP addr from
  first subnet.
- Update port with a second IP addr from second subnet.
- Update port to return to a single IP addr.
- Clean up resources (port, subnets, and network) via
  the tearDownClass method of the BaseNetworkTest class.

Change-Id: I0036cd7fe412f4a371027fc5c323a7bc3ccce5b9
Merge "Add network API test to create/update a port with 2 IP addresses"