Convert network/api.py to use FloatingIP object

This converts floating IP manipulation code in the network/api.py
module to use the FloatingIP object. Basically no testing of these
interfaces was being done, so this adds that as well.

Related to blueprint compute-manager-objects-juno

Change-Id: I6120dd4a32473ca9029832503444fbeae1ab0576
Merge "Convert network/api.py to use FloatingIP object"