Build autosubmitter plugin separately for stable-2.15

The master branch of the plugin is no longer compatible with
gerrit's stable-2.15 branch.

Change-Id: Id04cacf3370674eda365eb7f37aab5390b416fc7
diff --git a/jenkins/gerrit-bazel-plugin-autosubmitter.yaml b/jenkins/gerrit-bazel-plugin-autosubmitter.yaml
index 912a59d..499ea47 100644
--- a/jenkins/gerrit-bazel-plugin-autosubmitter.yaml
+++ b/jenkins/gerrit-bazel-plugin-autosubmitter.yaml
@@ -3,11 +3,12 @@
     jobs:
       - 'plugin-{name}-bazel-{branch}':
           branch:
+            - stable-2.15
             - stable-2.14
       - 'plugin-{name}-bazel-{branch}-{gerrit-branch}':
           branch:
             - master
           gerrit-branch:
             - master
-            - stable-2.15
+            - stable-2.16