Upstart `expect daemon` for aggregator

It seems that carbon-aggregator forks in a different way to carbon-cache.
Which without `expect daemon` causes the following to happen:

```
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
```