Rename schema job to be more clear about its purpose

The make target fails when it detects schema changes, not when schema is invalid.

Also update CONTRIBUTING.md to include information about zuul jobs.

tox.ini

@@ -6,7 +6,7 @@ envlist =
     api,
     ui,
     swagger,
-    validate-schema,
+    detect-schema-change,
 
 [testenv]
 ;basepython = python2.7
@@ -73,9 +73,9 @@ commands =
     make docker-compose-build
     make docker-compose-build-swagger
 
-[testenv:validate-schema]
+[testenv:detect-schema-change]
 deps =
     nodeenv
 commands =
     make docker-compose-build
-    make docker-compose-validate-schema
+    make docker-compose-detect-schema-change