panko create mongo user is not idempotent

When running kolla-ansible deploy more than once,
panko task fails to create user/db in mongodb.

TASK [panko : Creating Panko MongoDB database] *********************************
fatal: [localhost -> localhost]: FAILED! => {"changed": false, "cmd": ["docker", "exec", "-t", "mongodb", "mongo", "--host", "192.168.100.215:27017", "--eval", "db = db.getSiblingDB(\"panko\"); db.createUser({user: \"panko\", pwd: \"DLEZgyHsHh4zBzG0ZHKY77xva3qCWo8h1lmT6MG2\", roles: [ \"readWrite\", \"dbAdmin\" ]})"], "delta": "0:00:00.183725", "end": "2017-04-11 10:40:36.425679", "failed": true, "failed_when_result": true, "rc": 252, "start": "2017-04-11 10:40:36.241954", "stderr": "", "stdout": "MongoDB shell version: 2.6.12\r\nconnecting to: 192.168.100.215:27017/test\r\n2017-04-11T10:40:36.409+0100 Error: couldn't add user: User \"panko@panko\" already exists at src/mongo/shell/db.js:1086", "stdout_lines": ["MongoDB shell version: 2.6.12", "connecting to: 192.168.100.215:27017/test", "2017-04-11T10:40:36.409+0100 Error: couldn't add user: User \"panko@panko\" already exists at src/mongo/shell/db.js:1086"], "warnings": []}