OS-531 Do not return None in infoblox_db

python returns None by default, remove unnecessary code from
infoblox_db.

Change-Id: Id72c6c8798a7d3e41a5c2e2ef5a65ad462bf0c9a
Merge pull request #11 from roman-verchikov/bp/infoblox-plugin-neutron

OS-531 Do not return None in infoblox_db