monitoring: refine alarms on HHVM

Since we're not interested in single spikes of traffic/load, but rather
to sustained issues with HHVM performance on one server that may need
our attention; so we need 30% of the datapoints over a 10-minute
period to be affected before we send an alarm.

Change-Id: I11ae78ea91a4998076f81a4aaee2e4054e6d19a1
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>