Require backup user password to be changed on production

Currently backup user password is not specified in the list of variables required to be changed on production. This may result some users having insecure password `passw0rd` for a backup user in their production environment.

Author: rkiyanchuk

README.md

@@ -171,7 +171,7 @@ Required vars to change on production:
 ```yaml
 mongodb_user_admin_password
 mongodb_root_admin_password
-mongodb_root_backup_name
+mongodb_root_backup_password
 
 # if you use replication and authorization
 mongodb_security_keyfile