Add stable-2.15 for rabbitmq plugin

There is a stable-2.15 branch that is currently not supported
in gerritforge:
  https://gerrit.googlesource.com/plugins/rabbitmq/+/stable-2.15

Change-Id: I754c7718d26b5ed9c6c7e7b5bda396d3e8b34292
diff --git a/jenkins/gerrit-buck-bazel-plugin-rabbitmq.yaml b/jenkins/gerrit-buck-bazel-plugin-rabbitmq.yaml
index 9bc203c..820e4b6 100644
--- a/jenkins/gerrit-buck-bazel-plugin-rabbitmq.yaml
+++ b/jenkins/gerrit-buck-bazel-plugin-rabbitmq.yaml
@@ -10,4 +10,5 @@
       - 'plugin-{name}-bazel-{branch}':
           branch:
             - master
+            - stable-2.15
             - stable-2.14