icinga: Add graphite_series_threshold check

Checks points in a series individually for thresholds.
check_threshold only checks one set of metrics for thresholds,
rather than a series of metrics. This is painful for doing
things like check for free space in betalabs, etc.

Enables series checks for the betalabs graphite checks.

Change-Id: Ic61dd9fdddd581aff97f028d7a46e0496559f194