Add another check using graphite, small fixes.

This commit includes a few small things:
- Corrected the output of check_graphite (now we show two digits instead
  of an arbitrary number)
- Now check_graphite_anomaly is able to consider either only the upper
  bound or the lower bound of metrics (as this makes sense in most cases)
- Shortened the window of the requstats.5xx check to the last 15 minutes
- Added a first (experimental) anomaly detection on the error ratio.
- Made the 5xx check actually page us as it's usually fired off when
  something is wrong.

Change-Id: Idea08c16164e0280f861b920155f139395eacf8d
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>