Adding idempotency testing to the role func test

While not particularly important for this role,
this should establish a reusable pattern for adding
such testing to other roles.

Change-Id: Ia700cfd11c8f15e81f5f767dd723e5a323159919

tox.ini

@@ -162,7 +162,14 @@ commands =
                      -e "rolename={toxinidir}" \
                      -e "install_test_packages=True" \
                      {toxinidir}/tests/test.yml -vvvv
-
+    # Idempotency test
+    bash -c 'ansible-playbook -i {toxinidir}/tests/inventory \
+                     -e "rolename={toxinidir}" \
+                     -e "install_test_packages=True" \
+                     {toxinidir}/tests/test.yml \
+                     | grep -q "changed=0.*failed=0" \
+                     && (echo "Idempotence test: pass" && exit 0) \
+                     || (echo "Idempotence test: fail" && exit 1)'
 
 [testenv:linters]
 deps =