Add linters check to make sure site-variables is yaml

A YAML syntax error in site-variables was merged recently. Errors in
that file break basically everything, so add a check that the file at
least parses.

Change-Id: I9b2c47a0461275e1d53aabe387f0309719d46b77

tox.ini

@@ -41,6 +41,8 @@ commands =
   bash -c "find playbooks -type f -regex '.*.ya?ml' -exec \
     ansible-playbook --syntax-check -i {toxinidir}/tests/inventory \
     -e @tests/vars.yaml \{\} + > /dev/null"
+  # Make sure site-variables at least parses
+  python -c 'import yaml ; yaml.load(open("zuul/site-variables.yaml", "r"))'
 
 [testenv:gerrit]
 deps = PyYAML