[health_monitor] Use new style properties

* Use new property helpers
* Added properties to spec
* Added hm.intervals.prune_events property
* Added optional hm.email_interval property (for email plugin)
* Added optional hm.varz_enabled property (for /varz plugin)

Change-Id: I068db03bf49e049a797440e7306541a9f587d6d4
Merge "[health_monitor] Use new style properties"