Use stestr-3 when it's available
Use stestr-3 when it's available

python3-stestr provides stestr-3, With switch to fedora
it would be installed and used.

Change-Id: Ib9f23e592105d08281f400827416e6f126935b1d

openstack/puppet-openstack-integration,openstack/puppet-openstack-integration,openstack/puppet-openstack-integration