Tempest and master and neutron-tempest-plugin queens tag is not
compatible for stable/queens testing.
More details on https://bugs.launchpad.net/tempest/+bug/1859988
Also py2 drop will start required the python 3.6 version for Tempest
master tests.
Because stable/queens is EM, let's pin the Tempest for its tesing
instead of putting more effort to make Tempest master run-able.
Tempest 21.0.0 is supported Tag for Queens so let's use that for Queens
testing instead of Tempest master.
Closes-Bug: 1859988
[1] https://docs.openstack.org/tempest/latest/stable_branch_support_policy.html
Change-Id: Id4861830c46867b313a5a705fc4722ac13471777