Drop unused python3 classvar from jenkins::slave
Drop unused python3 classvar from jenkins::slave

The python3 class variable in jenkins::slave is no longer used by
the openstack_project module and should be removed for simplicity.

Change-Id: I59629767bdc53624aa84b518504d9f46e05e704d
Depends-On: Iac7ffde66a2d5d1a1361ed9bcadb9144e034f10f

open-switch/infra_puppet-jenkins,open-switch/infra_puppet-jenkins