OS-533 Add floating IPs API to IPAM

Adds more floating IP APIs to IPAM subsystem:
 * associate_floatingip() - called on floating IP associated with port;
 * disassociate_floatingip() - inverse of previous.

Change-Id: I92ce7618d478ec9ba005264dc8f618441ba2ab1f
OS-535/536 Fix flake8 issues and remove unused APIs

This fixes flake8 issues introduced by OS-532.
Includes unused method cleanup from base.py and neutron_ipam.py.

Change-Id: I8e19391a62d7848f93b43fe37a3b8a912ef97f8e
Merge pull request #7 from roman-verchikov/bp/infoblox-plugin-neutron

OS-533 Add floating IPs API to IPAM