Use python 3 on all nodes in multinode job

In neutron-tempest-plugin-dvr-multinode-scenario we had the same problem
as described in [1] for tempest multinode jobs. It was using python 3
on controller node but python 2 on all subnodes.
This patch changes that to use python3 on all nodes.

[1] https://bugs.launchpad.net/tempest/+bug/1853004

Change-Id: I9ef4daa821342d7462121a8bbde217ff5e0d7e9a
Related-Bug: #1853004
1 file changed