Allow custom configuration for apiserver host

[ghost]

Currently your system uses the kubectl_util.DefaultClientConfig() to retrieve the IP for the api-server, which may not work outside GCE in customized setups.

This PR provides optional flags to use a non-default api-server. It uses the same flags and borrows code from the kubernetes/dashboard project:
https://github.com/kubernetes/dashboard/blob/master/src/app/backend/client/apiserverclient.go
https://github.com/kubernetes/dashboard/blob/master/src/app/backend/dashboard.go#L31

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This change is

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[therc]

Reference to Dashboard

[bprashanth]

Is your goal to point at an apiserver that kubectl doesn't know about? because even outside a kubecluster, we should be looking up the default kubeconfig context, and pulling out the apiserver from there, at least the last time i checked

[aledbf]

@doug-clarifai this is not required. Please check this comment #1467 (comment)

[ghost]

@aledbf The comment you reference only allows a simple host name as an environment variable for the api server.

The goal of this change is to allow for more flexible configurations. kubernetes/dashboard added these flags to match kubectl and proxy: kubernetes/dashboard#1256 kubernetes/dashboard#905

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[k8s-github-robot]

@doug-clarifai PR needs rebase

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