Post-pool merge cleanup

(1) Removed references to designate-agent.
(2) Removed mdns references that are no longer needed.
(3) Ensure that create_domain creates soa first.
(4) Removed py26 from the default tox tests

Change-Id: Ic014fecac7f97fa88a45e3ac2a3e2d35299fc966
Merge "Post-pool merge cleanup"