We shouldn't be defining mongo role vars in the jenkins_worker role, can we just use the mongo role defaults?

from `roles/mongo/defaults/main.yml`
```
mongo_data_dir: "{{ COMMON_DATA_DIR }}/mongo"
mongo_log_dir: "{{ COMMON_LOG_DIR }}/mongo"
mongo_logpath: "{{ mongo_log_dir }}/mongodb.log"
mongo_dbpath: "{{ mongo_data_dir }}/mongodb"
```