Remove needless workaround in utils module

Workaround for bug in netaddr library has been removed due to the fix
was introduced in 0.7.6 and nova currently requires 0.7.12 or newer version.

Change-Id: If83cd8ee770d57547f4cfcdbba4a9fbfd55d5983
Related-Bug: #957708
Merge "Remove needless workaround in utils module"