upstart: use exec here so upstart can monitor the process and not just the spawning shell

Here is how the example docker upstart script (which seems to be bundled in the deb) runs docker:

```
$ sudo start docker
docker start/running, process 10150

$ ps uax | grep docker
root     10151  2.0  0.1 207452  6548 ?        Sl   10:23   0:00 /usr/bin/docker -d

$ ps aux | grep 10150
root     10150  0.0  0.0   4404   612 ?        Ss   10:23   0:00 /bin/sh -e /proc/self/fd/9
```

In this example, we can see that `upstart` believes that the pid it should be monitoring is 10150.  Upon inspection, we see that maps to the shell that spawns everything.  `docker` is actually on pid 10151.

If we use `exec`, things behave properly.  This is how things look on this branch:

```
ubuntu@docker-test5-pe1-prd:~$ sudo start docker
docker start/running, process 11394

ubuntu@docker-test5-pe1-prd:~$ ps aux | grep 11394
root     11394  2.2  0.1 272984  6380 ?        Ssl  10:28   0:00 /usr/bin/docker -d
```