Add /etc/default/docker support to upstart, too (mirroring sysvinit)

Now, we can have "/etc/default/docker" that contains:
```bash
DOCKER_OPTS="-D -H=unix:///var/run/docker.sock -H=tcp://127.0.0.1:4243"
```
... and both Upstart and SysV will use it correctly to start the docker daemon with the proper options.