Replace merge_configs with merge_yaml for merge mongodb.conf

Actually, 'mongodb.conf' is a yaml format configuration file. Do not use
merge_configs to merge it.

Change-Id: Id3c006df00c1e2d66472c2195781e01c640cab22
Signed-off-by: ZijianGuo <guozijn@gmail.com>

ansible/roles/mongodb/tasks/config.yml

@@ -27,7 +27,7 @@
     - Restart mongodb container
 
 - name: Copying over mongodb.conf
-  merge_configs:
+  merge_yaml:
     sources:
       - "{{ role_path }}/templates/{{ item.key }}.conf.j2"
       - "{{ node_custom_config }}/mongodb.conf"