Add configurable parameters for metric interval and event TTL

This pull requests adds two new configurable parameters, one for the interval at which metrics are sent and another for the event TTL when sending metrics.

Prior to these changes, the interval & TTL for all health metrics was hardcoded to 1. Since the interval was the same as the TTL, unless the system sent events and Riemann processed them exactly within 1 second, some services would incorrectly expire during the Riemann server's periodic expiration trigger and then get re-indexed again shortly afterwards.

The defaults are now interval=1 and TTL=5 so we have a bit of a grace period to have the events sent & processed.