Fixes not enough time for docker restart before continuing on to next provisioning step

Hi,

I'm getting an issue where docker doesn't have enough time to start before continuing on to the start service step:

```sh
➜  vagrant up
Bringing machine 'default' up with 'virtualbox' provider...
==> default: Importing base box 'ubuntu'...
  1 # -*- mode: ruby -*-
==> default: Matching MAC address for NAT networking...
==> default: Setting the name of the VM: harmony_default_1397580811827_63596
==> default: Clearing any previously set network interfaces...
==> default: Preparing network interfaces based on configuration...
    default: Adapter 1: nat
    default: Adapter 2: hostonly
==> default: Forwarding ports...
    default: 22 => 2222 (adapter 1)
==> default: Booting VM...
==> default: Waiting for machine to boot. This may take a few minutes...
    default: SSH address: 127.0.0.1:2222
    default: SSH username: vagrant
    default: SSH auth method: private key
    default: Warning: Connection timeout. Retrying...
==> default: Machine booted and ready!
==> default: Checking for guest additions in VM...
==> default: Configuring and enabling network interfaces...
==> default: Exporting NFS shared folders...
==> default: Preparing to edit /etc/exports. Administrator privileges will be required...
==> default: Mounting NFS shared folders...
==> default: Mounting shared folders...
    default: /vagrant => /Users/kruppel/Projects/zendesk/harmony
==> default: Running provisioner: docker...
    default: Installing Docker (latest) onto machine...
    default: Configuring Docker to autostart containers...
The following SSH command responded with a non-zero exit status.
Vagrant assumes that this means the command failed!

service docker start

Stdout from the command:



Stderr from the command:

stdin: is not a tty
start: Job is already running: docker
```

This is my proposed fix. What do you think?