Require netaddr>=0.7.6 to avoid UnboundLocalError

This issue is discussed at https://github.com/drkjam/netaddr/issues/2.
The bug in netaddr was fixed in release 0.7.6 on 13th Sep 2011.

Change-Id: Iea75a30963a09f9b0dd94138460bf041ab98fc66
Fixes: bug #1157058
Merge "Require netaddr>=0.7.6 to avoid UnboundLocalError"