This switches to python 3 as the remote python interpreter on
Debian/Ubuntu jobs, with CentOS 7 as the only exception using python 2.

Also switch to auto-detection of the interpeter except for CentOS 7,
which should be based on the one used by ansible-playbook (python 3).

Change-Id: Ie4aff6123dfc7267fe78f4bd736565fb72fe135e
Partially-Implements: python-3