Added package problemdaemon. All future problem daemons should be
registered by calling problemdaemon.register().
CLI interfaces will be automatically generated for all registered
problem daemons in the form of "--config.DAEMON_NAME"
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.
If --system-log-monitors or --custom-plugin-monitors are not
specified, npd gave us unclear message.
This patch adds the validation and clear error message.
