Add tox target for python3 test env

As devstack moves towards testing with python3 it would be potentially
useful to run devstack-gate without depending on python2. There are
likely other items that need porting/testing, but this is a start in
that process and makes sure we can test the test-matrix.py script with
python2 and 3.

Change-Id: Ida4906bfebc839bb7a0df11f53966a6b8c45689f

tox.ini

@@ -12,6 +12,12 @@ deps = -r{toxinidir}/test-requirements.txt
 commands =
   bash -c "./run-tests.sh"
 
+[testenv:py3-run-tests]
+basepython = python3
+deps = -r{toxinidir}/test-requirements.txt
+commands =
+  bash -c "./run-tests.sh"
+
 [testenv:bashate]
 deps=
   {env:BASHATE_INSTALL_PATH:bashate==0.5.0}