add and remove floating IPs to a NovaInstance

Change-Id: Id97243058d86384464e30dc80d8728a4fe97faaf

>>> from StackEnvironment import StackEnvironment
>>> se = StackEnvironment()
>>> se.nova.floating_ips.list()
[<FloatingIP fixed_ip=None, id=1, instance_id=None, ip=10.16.17.161, pool=public>, <FloatingIP fixed_ip=10.0.0.2, id=2, instance_id=ccfb6749-2354-43c5-a151-89bd343666f7, ip=10.16.17.162, pool=public>]
>>> instance = se.launch_instance(root_disk=('glance','242b6ad9-a2dc-4e1e-83ce-1fe0d7b0e492'))
>>> instance.floating_ips
[]
>>> instance.add_floating_ip()
>>> instance.floating_ips
[<FloatingIP fixed_ip=None, id=3, instance_id=None, ip=10.16.17.163, pool=public>]
>>> instance.remove_floating_ip(instance.floating_ips[0])
>>> instance.floating_ips
[]
>>>