node-problem-detector --version should not require monitors specify

[Lion-Wei]

Currently node-problem-detector --version command request monitor specified

./node-problem-detector --version
panic: Either --system-log-monitors or --custom-plugin-monitors is required

goroutine 1 [running]:
k8s.io/node-problem-detector/cmd/options.(*NodeProblemDetectorOptions).ValidOrDie(0xc000175900)
	/usr1/go/src/k8s.io/node-problem-detector/cmd/options/options.go:99 +0x13d
main.main()
	/usr1/go/src/k8s.io/node-problem-detector/cmd/node_problem_detector.go:65 +0x82

That's not necessary, and if we use node-problem-detector --version --system-log-monitors=./, that would be little weird.