[Bug] member cluster connect failed cause to kubecube startup failure

[weilaaa]

Describe the bug

member cluster connect failed cause to kubecube startup failure

2021-08-09T10:34:48.179+0800	error	kubernetes/kubernetes.go:81	problem new k8s client: Get "https://10.173.32.130:6443/api?timeout=32s": dial tcp 10.173.32.130:6443: connect: no route to host

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x40 pc=0x160106e]

goroutine 1034 [running]:
github.com/kubecube-io/kubecube/pkg/clients/kubernetes.NewClientFor.func1(0xc000df1280, 0x1ea3930, 0xc001c9e1c0)
	/workspace/pkg/clients/kubernetes/kubernetes.go:102 +0x2e
created by github.com/kubecube-io/kubecube/pkg/clients/kubernetes.NewClientFor
	/workspace/pkg/clients/kubernetes/kubernetes.go:101 +0x252

To Reproduce
Steps to reproduce the behavior:

1. teardown the member cluster
2. restart kubecube pod

Expected behavior
It's supposed to setting the status of cluster to abnormal until member cluster reconnect kubecube meanwhile refresh InternalCluster

Screenshots
If applicable, add screenshots to help explain your problem.

Desktop (please complete the following information):

• OS: [e.g. iOS]
• Browser [e.g. chrome, safari]
• Version [e.g. 22]

Smartphone (please complete the following information):

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Browser [e.g. stock browser, safari]
• Version [e.g. 22]

Additional context
Add any other context about the problem here.

[zhujf1989]

need to be fixed