Improve CPU checks

The previous check was being sampled every minute but only considered
the last sample. Here we replace the check with two checks, one that
runs a moving average over the previous 60 seconds every minute which
will tell us if a machine is under sustained load. We also have a
check averaged over the previous 10 seconds every 10 seconds which will
highlight short sharp spikes.
Merge pull request #181 from alphagov/improve-cpu-checks

Improve CPU checks