check_graphite_metric: add parameter for metric check timeout

Add a new parameter to define the amount of time to fetch metric values for.

Useful to define time frames for which a "null" value is collected, for example.

The check will query graphite with "&from=-Nminutes" to specify how much time back is
fetched and searched for values. Only the last non null value is then evaluated for
alerts.

While that's not new, this commit enables us to specify longer than usual times which
can be used in combination with other strategies to find out, for example, whether it
is too long since the last known value in the metric.