varnish-cps: moving backend threshold definition to VarnishHealthCheck

- also logging a warning if the threshold will never be met by the time period of the graphite check
Improvements to the varnish-cps unhealthy period logic:

- changed graphite call to summarize time buckets with `alignToFrom=true`
-- this improves the accuracy of the unhealthy period (the final datapoint now applies for the whole time bucket interval rather than just a part of it)
-- this also simplifies the calculation as the number of time buckets is no longer subject ot fencepost errors

- For critical alerts we now distinguish between those that have gone above the user defined threshold and those that have been failing for at least the entire duration of the check interval
- Improved the time duration display format on critical alerts