Ensure openshift-sdn and flannel can't be used at the same time

roles/openshift_common/tasks/main.yml

@@ -1,4 +1,8 @@
 ---
+- fail:
+    msg: Flannel can not be used with openshift sdn
+  when: openshift_use_openshift_sdn | bool and openshift_use_flannel | bool
+
 - name: Set common Cluster facts
   openshift_facts:
     role: common