Test for IP Address attachment

[almahmoud]

I'm opening this issue to continue the conversation started by @nuwang here galaxyproject/cloudlaunch#163 about adding a test for IP Address attachment.
Given that we don't have a "Network Interface" object in CloudBridge, it might be hard to make a test that specifically caters to the case where an IP is attached but has no associated instance. A potential way to go about this could be to add a test where an IP is attached to an instance, then the instance is terminated but not detached from the IP, thus the IP would stay attached to the Interface without an associated Instance, then make sure that it is still detected as "in use" and cannot be attached. This should work for AWS, and most likely would work for Azure with a nic, however I am not sure whether it would work for OpenStack.