Improve proxy_vhost 4/5xx sensu checks

- Measure on the total number of matching requests over the previous 1
  minute. `keepLastValue` means that the line doesn't drop to zero until
  all non-zero records have fallen out of the window. It is also a bit
  harder to understand.
- Set graphite sensu checks to be empty if `$sensu_check` is switched
  off. Apparently this is the only way sensu checks can be disabled
  right now.
Add more lenient 4xx limits for monitoring apps
Merge pull request #180 from alphagov/improve-proxy-vhost-sensu-checks

Improve proxy vhost sensu checks