jenkins/config: add JCASC dropin for jms-messaging-plugin

This configures the JMS messaging plugin to connect to the public
endpoint of the Fedora messaging bus.

The tricky thing is passing in the keystores. The plugin does
not support using Jenkins credentials[[1]] so we can't leverage
kubernetes-credentials-provider.

We could do it the old way, which is to mount the secret into the
Jenkins pod, but since d6d1f61, CoreOS CI now uses the exact same
`jenkins.yaml` manifest as the production pipeline and we don't want to
bind mount it there.

Instead, we hack around this by just baking the keystores in the Jenkins
image at `$JENKINS_HOME/jms-messaging-stores`.

[1]: jenkinsci/jms-messaging-plugin#263

jenkins/config/jms.yaml

@@ -0,0 +1,16 @@
+unclassified:
+  jmsProviders:
+    configs:
+      - rabbitMq:
+          name: "Fedora Messaging"
+          hostname: "rabbitmq.fedoraproject.org"
+          portNumber: 5671
+          virtualHost: "/public_pubsub"
+          exchange: "amq.topic"
+          authenticationMethod:
+            sslCertificate:
+              username: "fedora"
+              keystore: "${JENKINS_HOME}/jms-messaging-stores/keystore.jks"
+              keypwd: "fedora"
+              truststore: "${JENKINS_HOME}/jms-messaging-stores/truststore.jks"
+              trustpwd: "fedora"