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/health/healther.go

@@ -38,5 +38,7 @@ func (h *health) poll() {
 }
 
 // Plugins that implements the Healther interface.
-// TODO(miek): none yet.
-var healthers = map[string]bool{}
+var healthers = map[string]bool{
+	"erratic":    true,
+	"kubernetes": true,
+}