ensure fernet-keys folder permissions

[AlvaroVega]

Just for case of docker is using a volume for store fernet-keys (to enable distribute fernet keys abroad keystone nodes)

iot-keystone:
  container_name: iot-keystone
  hostname: iot-keystone
  image: telefonicaiot/fiware-keystone-spassword:latest
  links:
    - iot-mysql
  ports:
    - "5001:5001"
  environment:
    - LOG_LEVEL=DEBUG
  volumes:
    - ${PATH_LOG}/keystone:/var/log/keystone
    - ${PATH_KEYSTONE_KEYS)/keystone-keys:/etc/keystone/fernet-keys
  command: -dbhost iot-mysql -default_pwd XXX -mysql_pwd iotonpremise
  log_driver: json-file
  log_opt:
    max-size: "250m"

includes doc about issue #163

[fgalan]

Do we have CNR in thi repo? Should something be added to it about the change done in this PR?

[fgalan]

LGTM