add python 3.6 unit test job

This is a mechanically generated patch to add a unit test job running
under Python 3.6 as part of the python3-first goal.

See the python3-first goal document for details:
https://governance.openstack.org/tc/goals/stein/python3-first.html

Note that python36 fails installing nodejs-legacy since bionic does not
include it. Install nodejs-legacy only on Xenial.

Change-Id: Id8fbe2d5188054924eacb74bcfc52b49fd8a1d84
Story: #2002586
Task: #24327

.zuul.yaml

@@ -4,6 +4,7 @@
       - openstack-python-jobs
       - publish-openstack-docs-pti
       - openstack-python35-jobs
+      - openstack-python36-jobs
     periodic:
       jobs:
         - legacy-periodic-package-stackviz-element

bindep.txt

@@ -2,5 +2,5 @@
 # see http://docs.openstack.org/infra/bindep/ for additional information.
 
 nodejs [test]
-nodejs-legacy [platform:dpkg test]
+nodejs-legacy [platform:ubuntu-xenial test]
 npm [test]