Fix used python versions for CI

Change-Id: Ib3ce073e260c1387f7d26b7d994da2fdc8077e06

.zuul.d/docker-jobs.yaml

@@ -28,7 +28,7 @@
 - job:
     name: rally-openstack-docker-build
     parent: build-docker-image
-    nodeset: ubuntu-bionic
+    nodeset: ubuntu-focal
     run: tests/ci/playbooks/docker-build-and-check.yaml
     post-run: tests/ci/playbooks/fetch-html-and-json-reports.yaml
     timeout: 1800
@@ -44,7 +44,7 @@
 - job:
     name: rally-openstack-docker-build-and-push
     parent: build-docker-image
-    nodeset: ubuntu-bionic
+    nodeset: ubuntu-focal
     run: tests/ci/playbooks/docker-build-check-and-push.yaml
     post-run: tests/ci/playbooks/fetch-html-and-json-reports.yaml
     timeout: 1800

tox.ini

@@ -50,6 +50,7 @@ commands = {posargs}
 commands = oslo_debug_helper -t tests {posargs}
 
 [testenv:functional]
+basepython = python3
 sitepackages = True
 commands =
   find . -type f -name "*.pyc" -delete