Update netaddr to 0.7.12 version for IPv6

Netaddr below 0.7.12 can not check IPv6 subnets for overlapping
It causes neutron to fail.

Change-Id: I139f51d2ef49d27e6ae29fe551df31c981653e7c
Closes-Bug: 1364166
Merge "Update netaddr to 0.7.12 version for IPv6"