plugin/health: implement dyn health checks (#1214)

Implement health.Healther in erratic and kubernetes plugin. The
kubernetes' healtcheck is only performed on startup - i.e. turn
healthy after the initial loading.

Erratic follow the drop count: every query%drop turns the healthcheck
unhealthy.

Fixes: #985

plugin/erratic/README.md

@@ -25,6 +25,10 @@ erratic {
 * `delay`: delay 1 per **AMOUNT** of queries for **DURATION**, the default for **AMOUNT** is 2 and
   the default for **DURATION** is 100ms.
 
+## Health
+
+This plugin implements dynamic health checking. For every dropped query it turns unhealthy.
+
 ## Examples
 
 ~~~ corefile