kubeconfig check should support only specified token when import cluster

[huxiaoliang]

Refer to code https://github.com/kubernetes/client-go/blob/master/rest/config.go#L501， we can see that kubeconfig support only specified token to access k8s cluster, but UI ask CertFile MUST be specified when import cluster, please UI enhance here to align with native k8s behavior

[huxiaoliang]

@jo-hnny could you please take a look this issue?

[huxiaoliang]

UI validation rule:
https://github.com/tkestack/tke/blob/master/api/platform/validation/clustercredential.go#L48-L60