Glusterfs: use automatic mtime-based split-brain resolution

Change-Id: I9975d30dd57721a8703d4144ba3d6c1909a789c1
diff --git a/glusterfs/server/volume/mongodb.yml b/glusterfs/server/volume/mongodb.yml
index a9c69f7..f694ad7 100644
--- a/glusterfs/server/volume/mongodb.yml
+++ b/glusterfs/server/volume/mongodb.yml
@@ -15,3 +15,4 @@
             network.remote-dio: On
             diagnostics.client-log-level: WARNING
             diagnostics.brick-log-level: WARNING
+            cluster.favorite-child-policy: mtime