Retrieve dataset name from mountpoint instead of commandline option

This patch will:
- check on which filesystem the directory '/var/lib/docker' is located                                                                                                                                              
- if its a zfs get the dataset name from /proc/mounts

reason:
- Some distribution have no configuration to pass commandline options to docker:
  ex: ./contrib/init/systemd/docker.service
  docker should work without editing init configuration
- this way one also could create an additional dataset where all docker volumes are included ('data/docker' instead 'data'), which allow to set additional zfs options for docker volumes through inheritance
- currently docker fails with an error message if libzfs is available, when it tries the zfs driver, because the zfs pool 'docker' is not available