With multi-ansible support, the location of the zuul plugins/modules
has moved.
Also, handle the sort_keys argument which is new in pyyaml 5.1.
Set it to False to achieve the old behavior.
Also use python3 by default for tox tests on this repo (note,
one env still explicitly requests 2.7).
Change-Id: I8e2cf1f2276bca5a912fed52062628b3b9d0d6d9