{Reviewer: Matt} Per-call SNMP stats checks

Matt,

This is a simpler approach to SNMP that just verifies that low-water-marks, high-water-marks and averages are in the right order, and that latency is below a certain threshold (I've said 250ms).

Some restrictions:
* It only works with Bono nodes - we don't necessarily know the public hostname of Sprout nodes (e.g. it'll be "sprout.DOMAIN" for a manual install and "sprout-1.DOMAIN" for Chef
* it uses the deployment domain, which may mean it targets a different Bono to the one the call went through. I'm not sure that we can easily get round this (maybe if we parse the Via headers?) or that it's worth doing.