Doesn't start after installation

[SRU Description]

[Impact]
This bug affect the start of the docker daemon, as it isn't started after installation.
Upgrade is broken also when using a direct backport from wily.

Some ajustment had to be done in docker.io.postinst to correct this upgrade behavior.

[Test Case]
start a fresh installed vivid
sudo apt-get update && sudo apt-get install docker.io
sudo systemctl status docker.socket show that the daemon is not started.

Actual results:
● docker.socket - Docker Socket for the API
   Loaded: loaded (/lib/systemd/system/docker.socket; enabled; vendor reset:enabled)
   Active: inactive (dead)
   Listen: /var/run/docker.sock (Stream)

To test this proposed package
you can on a fresh installation each time:

1-Install the proposed package directly, and check that docker.socket service is active

  sudo systemctl status docker.socket
  Expected results:
    ● docker.socket - Docker Socket for the API
       Loaded: loaded (/lib/systemd/system/docker.socket; enabled; vendor preset: enabled)
       Active: active (running) since Fri 2015-06-26 16:05:46 UTC; 19s ago
       Listen: /var/run/docker.sock (Stream)

    sudo docker version
  Expected results:
    Client version: 1.6.2
    Client API version: 1.18
    Go version (client): go1.3.3
    Git commit (client): 7c8fca2
    OS/Arch (client): linux/amd64
    Server version: 1.6.2
    Server API version: 1.18
    Go version (server): go1.3.3
    Git commit (server): 7c8fca2
    OS/Arch (server): linux/amd64

2-On a fresh vivid install, install docker-1.5.0,
    sudo docker version:
  expected results:
    Client version: 1.5.0
    Client API version: 1.17
    Go version (client): go1.3.3
    Git commit (client): a8a31ef
    OS/Arch (client): linux/amd64
    FATA[0000] Get http:///var/run/docker.sock/v1.17/version: dial unix /var/run/docker.sock: no such file or directory. Are you trying to connect to a TLS-enabled daemon without TLS?

  then upgrade to proposed package, and check if docker service is active.

    sudo systemctl status docker.socket,
  expected results:
  ● docker.socket - Docker Socket for the API
     Loaded: loaded (/lib/systemd/system/docker.socket; enabled; vendor preset: enabled)
     Active: active (running) since Fri 2015-06-26 16:15:18 UTC; 23s ago
     Listen: /var/run/docker.sock (Stream)

    sudo docker version;
  Expected results:
    Client version: 1.6.2
    Client API version: 1.18
    Go version (client): go1.3.3
    Git commit (client): 7c8fca2
    OS/Arch (client): linux/amd64
    Server version: 1.6.2
    Server API version: 1.18
    Go version (server): go1.3.3
    Git commit (server): 7c8fca2
    OS/Arch (server): linux/amd64

3-Install docker-1.5.0,
    start service manually:
    sudo systemctl start docker.socket && sudo systemctl status docker.socket
  Expected results:
    ● docker.socket - Docker Socket for the API
       Loaded: loaded (/lib/systemd/system/docker.socket; enabled; vendor preset: enabled)
       Active: active (listening) since Fri 2015-06-26 16:20:41 UTC; 7ms ago
       Listen: /var/run/docker.sock (Stream)

  then upgrade to proposed package, and check if docker service is active.

    sudo systemctl status docker.socket,
  expected results:
  ● docker.socket - Docker Socket for the API
     Loaded: loaded (/lib/systemd/system/docker.socket; enabled; vendor preset: enabled)
     Active: active (running) since Fri 2015-06-26 16:15:18 UTC; 23s ago
     Listen: /var/run/docker.sock (Stream)

    sudo docker version;
  Expected results:
    Client version: 1.6.2
    Client API version: 1.18
    Go version (client): go1.3.3
    Git commit (client): 7c8fca2
    OS/Arch (client): linux/amd64
    Server version: 1.6.2
    Server API version: 1.18
    Go version (server): go1.3.3
    Git commit (server): 7c8fca2
    OS/Arch (server): linux/amd64

[Regression Potential]
This fix reset the saved state of systemd docker.service and docker.socket units,
so it may affect a user that has installed docker.io (1.5.0~dfsg1-1ubuntu2),
then rebooted or manually started the service, then has manually shutdown the service.
The last state will be reset.

It is worth noting than since docker.io-1.6.0~rc4~dfsg1-1,
the docker.service is marked on-boot, so docker.socket is not so usefull anymore.
It is retained for minimizing divergence with upstream.

[Original Bug Description]

I need to manually invoke "systemctl start docker.socket".