Add gerrit.googlesource.com to zuul using git driver

So that we can make gerrit build jobs using zuul to do the cloning
and not git modules inside of the jobs themselves, add gerrit's
git server as a git driver source.

Change-Id: I4a64e6627805d568030d3864f3d331a4e8f9ab8b

hiera/group/zuul-executor.yaml

@@ -9,3 +9,7 @@ zuul_connections:
 
   - name: 'github'
     driver: 'github'
+
+  - name: 'googlesource'
+    driver: 'git'
+    baseurl: https://gerrit.googlesource.com/

hiera/group/zuul-merger.yaml

@@ -39,3 +39,7 @@ zuul_connections:
 
   - name: 'github'
     driver: 'github'
+
+  - name: 'googlesource'
+    driver: 'git'
+    baseurl: https://gerrit.googlesource.com/

hiera/group/zuul-scheduler.yaml

@@ -29,6 +29,10 @@ zuul_connections:
     driver: 'github'
     app_key: '/etc/zuul/github.key'
 
+  - name: 'googlesource'
+    driver: 'git'
+    baseurl: https://gerrit.googlesource.com/
+
 gearman_server_ssl_cert: |
   -----BEGIN CERTIFICATE-----
   MIIEYTCCA0mgAwIBAgIJAKkAn3gh0LBPMA0GCSqGSIb3DQEBCwUAMIG5MQswCQYD