Require dnspython>=1.12.0

The Designate project requires a particular bugfix included in
dnspython 1.12.0 commit 2db54ba975ed5be22f92af105bb2bd6b0ccbbef1.

This buxfix allows for correctly TSIG signing a DNS response
packet.

Change-Id: I4c66dbaf38f4771301106cb4349b64037115972a
Merge "Require dnspython>=1.12.0"