Force carbon-aggregator to run in foreground

This is the same dirty hack applied to carbon-cache in #13. Otherwise the
following happens, and variations of `expect` cause it to hang :(

```
root@graphite-1:~# start carbon-aggregator
carbon-aggregator start/running, process 23813
root@graphite-1:~# status carbon-aggregator
carbon-aggregator stop/waiting
root@graphite-1:~# cat /var/log/upstart/carbon-aggregator.log
Starting carbon-aggregator (instance a)
carbon-aggregator (instance a) is already running with pid 23491
carbon-aggregator (instance a) is already running with pid 23491
carbon-aggregator (instance a) is already running with pid 23491
carbon-aggregator (instance a) is already running with pid 23491
carbon-aggregator (instance a) is already running with pid 23491
carbon-aggregator (instance a) is already running with pid 23491
carbon-aggregator (instance a) is already running with pid 23491
carbon-aggregator (instance a) is already running with pid 23491
carbon-aggregator (instance a) is already running with pid 23491
carbon-aggregator (instance a) is already running with pid 23491
```