Use main.yaml, not .yml

There are over 490 .yaml files but only a few .yml, let's rename to be
consistent.

Add a test to block .yml files.

Change-Id: I2f1354de82f231154d926b51d9812b1e9c1a6202

tox.ini

@@ -48,6 +48,8 @@ commands =
     ansible-playbook --syntax-check -i {toxinidir}/tests/inventory \{\} + > /dev/null"
   {toxinidir}/tools/check_jobs_documented.py
   {toxinidir}/tools/update-test-platforms.py
+  bash -c "(( $(find playbooks -name *.yml | wc -l) == 0)) || \{ echo 'Use .yaml'; exit 1; \}"
+  bash -c "(( $(find roles -name *.yml | wc -l) == 0)) || \{ echo 'Use .yaml'; exit 1; \}"
   bash -c "git diff --quiet || \{ echo 'ERROR: git in dirty status, reporting as failure'; exit 1; \}"
 
 [testenv:venv]