icinga: Fix anomaly detection checks

Changed the check for reqstats.5xx to use the data and not the ratio to
requests as although formally more correct it's much more unstable than
the base check. Also, check_graphite now requests the holt-winters
anomaly bands with 5 delta confidence, which has proven to be much
nearer to detect a real anomaly with less false positives.

Change-Id: I8b06b0b139b292e57b97417c0f880ec660fbbf2d
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>