Added gerrit auto abandon config

Change-Id: I04bd46786461a2c96260ecb8edbb9f37f99d9275
diff --git a/gerrit/server/single.yml b/gerrit/server/single.yml
index 6704f97..c279f00 100644
--- a/gerrit/server/single.yml
+++ b/gerrit/server/single.yml
@@ -52,6 +52,8 @@
         password: ${_param:postgresql_gerrit_password}
         pool_limit: 250
         pool_max_idle: 16
+      change_cleanup:
+        abandon_after: 3months
   postgresql:
     server:
       version: "9.5"