Build high-availability plugin on GitHub for stable-2.15

The stable-2.15 branch is functionally broken on gerrit-review
because the inability to safely align change index post NoteDb
migration.

A series of changes is currently under review. However, until
they are merged, it is safer to provide to the Gerrit Community
an implementation that is tested and is working in production
for Gerrit v2.15.x/NoteDb.

Change-Id: I8508e2b8000cb82d326824cfcd93e8cb73beef90
diff --git a/jenkins/gerrit-buck-bazel-plugin-high-availability.yaml b/jenkins/gerrit-buck-bazel-plugin-high-availability.yaml
index 9547f3b..353cdcb 100644
--- a/jenkins/gerrit-buck-bazel-plugin-high-availability.yaml
+++ b/jenkins/gerrit-buck-bazel-plugin-high-availability.yaml
@@ -12,3 +12,4 @@
       - 'plugin-{name}-gerritforge-{branch}':
           branch:
             - stable-2.14
+            - stable-2.15