Make most platform jobs run with python3
Make most platform jobs run with python3

python2.7 will be EOL soon, let's test on python3 instead.
Exclude CentOS 7, as there is no python3 easily available.

Change-Id: I24d8812c0c37b6d376fd5ae38067513bb62a2804

dtroyer/devstack,nawawi/openstack,nawawi/openstack,openstack-dev/devstack,openstack-dev/devstack,dtroyer/devstack