Move apiserver-reporting logic into k8s_exporter.

Added CLI option "enable-k8s-exporter" (default to true). Users can use
this option to enable/disable exporting to Kubernetes control plane.

This commit also removes all the apiserver-specific logic from package
problemdetector.

Future exporters (e.g. to local journald, Prometheus, other control
planes) should implement types.Exporter interface.

pkg/types/types.go

@@ -107,3 +107,9 @@ type Monitor interface {
 	// Stop stops the log monitor.
 	Stop()
 }
+
+// Exporter exports machine health data to certain control plane.
+type Exporter interface {
+	// Export problems to the control plane.
+	ExportProblems(*Status)
+}