We get deprecation warnings from ansible about use
of python2 on xenial hosts. Rather than setting
ansible_python_interpreter to python3 on a host by
host basis, set it globally to python3.
Set it to python for the one host that's too old,
refstack.openstack.org, which is running on trusty
which only has python3.4.
Change-Id: I4965d950c13efad80d72912911bc7099e9da1659