OS-532 Add docstrings to IPAM base APIs

As part of community review docstrings must be added to IPAM base APIs.

Change-Id: I71fb92c61cf688b251b553ef4d27020b352e3d7f
Merge pull request #4 from roman-verchikov/bp/infoblox-plugin-neutron

OS-532 Add docstrings to IPAM base APIs