tests: Make python version configurable

This is good if you want to run a debugger shell (like winpdb) instead
of just running vanilla python.

Change-Id: I21986bca22088100bb88deba32e17e7dcead5b5d
Signed-off-by: Saggi Mizrahi <smizrahi@redhat.com>
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/11399
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>