scenario tests: use py2 on <=train branches
scenario tests: use py2 on <=train branches

Those tests are used on the stable branches too, where they should
run using Python 2 (there is a specific py3 test there).

Change-Id: I64b3e6f0edaa2cc166ebeccbba5cd2cfffb452b0

openstack/sahara-tests,openstack/sahara-tests,openstack/sahara-tests,openstack/sahara-tests