Ordered the lookup of machine IDs to guard against randomly ordered map ...

In Go v1.3, map ordering was intentionally randomized to discourage relying on maps which were coincidentally ordered how expected. This corrects a non-deterministic bug in one of the tests which was caused by this change to Go.
As we're unconditionally adding the ids, we could use `make([]string, len(m.machines)` and set with `machineIds[i] = machineId` - not sure which style we prefer.

I would omit the 'sorted' prefix from the variable name though, as it becomes sorted, but is not initialised as such.