[ussuri][goal] Drop python 2.7 support and testing

OpenStack is dropping the py2.7 support in ussuri cycle.

Watcher is ready with python 3 and ok to drop the
python 2.7 support.

Complete discussion & schedule can be found in
- http://lists.openstack.org/pipermail/openstack-discuss/2019-October/010142.html
- https://etherpad.openstack.org/p/drop-python2-support

Ussuri Communtiy-wide goal:
https://governance.openstack.org/tc/goals/selected/ussuri/drop-py27.html

Depends-On: https://review.opendev.org/#/c/693631/

Change-Id: I603c6d2c22779e8ef2e70eb6369fc521a77c9c3a

playbooks/legacy/grenade-devstack-watcher/run.yaml

@@ -40,7 +40,7 @@
 
           export DEVSTACK_GATE_TEMPEST_NOTESTS=1
           export DEVSTACK_GATE_GRENADE=pullup
-
+          export DEVSTACK_GATE_USE_PYTHON3=True
           export BRANCH_OVERRIDE=default
           if [ "$BRANCH_OVERRIDE" != "default" ] ; then
               export OVERRIDE_ZUUL_BRANCH=$BRANCH_OVERRIDE