Build lfs plugin on its own stable-3.6 branch

Also build plugin on stable-3.6 branch against gerrit stable-3.7 branch.
Also add build against gerrit stable-3.8 branch.

Also hoist extra-plugins to the job definition to avoid the attribute
duplication per job base.

Change-Id: I36f9f4e94c5b4d2f08e33b78200ec0a87256b2fa
diff --git a/jenkins/gerrit-bazel-plugin-lfs.yaml b/jenkins/gerrit-bazel-plugin-lfs.yaml
index 1e9c6b5..a276357 100644
--- a/jenkins/gerrit-bazel-plugin-lfs.yaml
+++ b/jenkins/gerrit-bazel-plugin-lfs.yaml
@@ -1,11 +1,18 @@
 - project:
     name: lfs
+    extra-plugins: ''
     jobs:
+      - 'plugin-{name}-bazel-{branch}':
+          branch:
+            - master
+            - stable-3.6
       - 'plugin-{name}-bazel-{branch}-{gerrit-branch}':
-          extra-plugins: ''
           branch:
             - master
           gerrit-branch:
-            - master
-            - stable-3.7
+            - stable-3.8
+      - 'plugin-{name}-bazel-{branch}-{gerrit-branch}':
+          branch:
             - stable-3.6
+          gerrit-branch:
+            - stable-3.7