'docker daemon' should be replaced with 'dockerd'

[twang2218]

I ran into an issue during my test on Docker 1.13-rc1. I created a DigtialOcean docker host via docker-machine command:

docker-machine create --engine-install-url "https://test.docker.com/" --engine-storage-driver overlay2 d1

After it's been created found it's failed to run docker command, and the systemd status is following:

root@d1:~# systemctl status docker
● docker.service
   Loaded: loaded (/etc/systemd/system/docker.service; enabled; vendor preset: enabled)
   Active: active (running) since Mon 2016-11-14 01:57:22 UTC; 6s ago
 Main PID: 1657 (docker)
    Tasks: 4
   Memory: 3.7M
      CPU: 19ms
   CGroup: /system.slice/docker.service
           └─1657 /usr/bin/docker daemon -H tcp://0.0.0.0:2376 -H unix:///var/run/docker.sock --storage-driver overlay2 --tlsverify --tlscacert /etc/docker/ca.pem --tlscert /etc/docker/server.p

Nov 14 01:57:22 d1 systemd[1]: Started docker.service.
Nov 14 01:57:22 d1 docker[1657]: Command "daemon" is deprecated, and will be removed in Docker 1.16. Please run `dockerd` directly.

As you can see, it's /usr/bin/docker daemon, which should be /usr/bin/dockerd.

It's a bug, moby/moby#28368, in docker which caused the failure to boot up, however, docker-machine should use dockerd anyway, as the docker daemon is deprecated since 1.12.