Add tox python3 overrides

Change-Id: I06915601774b581b377e8c048c819e045d26df22

tox.ini

@@ -48,6 +48,7 @@ basepython = python3
 commands = {posargs}
 
 [testenv:debug]
+basepython = python3
 commands = oslo_debug_helper -t tests {posargs}
 
 [testenv:debug35]
@@ -66,10 +67,12 @@ commands = {toxinidir}/tests/ci/cover.sh {posargs}
 
 
 [testenv:genconfig]
+basepython = python3
 commands =
     oslo-config-generator --config-file etc/rally/rally-config-generator.conf
 
 [testenv:requirements]
+basepython = python3
 deps = requests[security]
 install_command = pip install -U {opts} {packages}
 commands = python {toxinidir}/tests/ci/sync_requirements.py {posargs}
@@ -95,4 +98,5 @@ deps = bindep
 commands = bindep
 
 [testenv:self]
+basepython = python3
 commands = {toxinidir}/tests/ci/rally_self_job.sh {toxinidir}/rally-jobs/self-rally.yaml