Fix the GOMAXPROCS warning for Upstart-based systems

**Description**

Improvements to the Upstart startup script:

* Set default value of `GOMAXPROCS=2`
* Make `GOMAXPROCS` configurable via `/etc/default/consul`
* Follow Upstart idioms like using `setuid`/`setgid`

**Testing Done**
Tested on Ubuntu 12.04:
* Saw no more warnings about `GOMAXPROCS` in `/var/log/upstart/consul.log` using the default config
* If I set `GOMAXPROCS=1` in `/etc/default/consul`, the warning comes back so the sourcing of that file is working.
* Saw that `consul` was started with the correct uid/gid :

       ```
$ ps -eafww  | grep consul
consul   15473     1  0 17:20 ?        00:00:03 /usr/local/bin/consul agent -config-dir /etc/consul
```
* Verified that `service consul restart` and `service consul reload` both work as expected.