This change switches the CI jobs to use python 3 for local execution of
the kolla-ansible commands.

For upgrades, we use python 2 for the previous (Train) deploy, then
reinstall using python 3 for the (Ussuri) upgrade.

NOTE: This is separate from the python interpreter used on remote hosts,
which is configured via ansible_python_interpreter.

Partially Implements: blueprint python-3
Related: blueprint drop-py2-support

Change-Id: I5bdc165f68b7bde1f9ef30fe8216f2a44e6d4706