Fix incorrect use of signals in upstart

FAO: @crxpandion 

[Mounted is an established signal](http://upstart.ubuntu.com/cookbook/#mounted) which results in two things:

 * we were triggering it needlessly
 * we were not filtering it in our `spade.conf` and as a result were starting spade prior to `/mnt` being mounted

This commit fixes these things, the former is benign, the latter was resulting in us still writing to the root partition, which should be fixed now as we establish the symlink and then trigger the formatting/mounting, which in turn will trigger the `mounted` event with `MOUNTPOINT` set to `/mnt`, which we then use to start spade.
It's not required, but it is used to inform documentation - i.e. when I general the dependency graph it knows where to draw arrows for our tools.
![upstart](https://cloud.githubusercontent.com/assets/260015/4816883/bb7b8462-5ee3-11e4-8115-fe9cac199e1f.png)