Refactor gerrit master job

Make a "base" version of the gerrit master job with no file matcher
so that we can use it in other repositories (eg, gerrit, zuul).
Inherit from it with the original name to add the file matcher back.

Change-Id: I4e428b44dd82f8dba08b219cbf8407969c6436b1

.zuul.yaml

@@ -380,8 +380,11 @@
 
 # Gerrit master jobs
 - job:
-    name: system-config-build-image-gerrit-master
-    description: Build a gerrit image.
+    name: system-config-build-image-gerrit-master-base
+    description: |
+      Build a gerrit image.
+
+      This job has no files matchers so it can be used in other repos.
     parent: system-config-build-image
     pre-run:
       - playbooks/zuul/gerrit/repos.yaml
@@ -418,6 +421,14 @@
         - gitiles
         - plugin-manager
         - webhooks
+
+- job:
+    name: system-config-build-image-gerrit-master
+    description: |
+      Build a gerrit image.
+
+      This job is used in system-config.
+    parent: system-config-build-image-gerrit-master-base
     files: &gerrit_files_master
       - docker/bazel/.*
       - docker/gerrit/base/.*