Use python3 for functional tests

Switch to python3 for the tox basepython and for the devstack setup for
functional tests to fulfill the python3-first goal[1]. We still have
unit test coverage for python2.

[1] https://governance.openstack.org/tc/goals/stein/python3-first.html

Change-Id: If82196ce6a2fffe6a43bb59077c6bdd712276958

.zuul.yaml

@@ -7,6 +7,8 @@
       - openstack/python-keystoneclient
     vars:
       tox_envlist: functional
+      devstack_localrc:
+        USE_PYTHON3: True
 
 - project:
     templates:

tox.ini

@@ -50,6 +50,7 @@ basepython = python3
 commands = oslo_debug_helper -t keystoneclient/tests {posargs}
 
 [testenv:functional]
+basepython = python3
 setenv = {[testenv]setenv}
          OS_TEST_PATH=./keystoneclient/tests/functional
 passenv = OS_*