add multi-pr presubmit testing

https://docs.ci.openshift.org/docs/how-tos/multi-pr-presubmit-testing/
Not 100% sure this will be useful to OADP unless we can build
the containers from the other repos and create the override

Author: weshayutin

Diff for: core-services/prow/02_config/openshift/oadp-operator/_pluginconfig.yaml

@@ -10,3 +10,9 @@ plugins:
   openshift/oadp-operator:
     plugins:
     - approve
+external_plugins:
+  openshift:
+  - endpoint: http://multi-pr-prow-plugin
+    events:
+    - issue_comment
+    name: multi-pr-prow-plugin