Expose Kubernetes rest config

[nicolaferraro]

I'd like to expose the rest configuration in order to be able to create custom clients e.g. a generic (non resource-scoped) dynamic client or a specific Openshift client in order to access some sub-resources (in my case, processing Openshift templates).

Can it be exposed?

[maleck13]

This would be very useful for initializing other clients. At the moment I have recreate the code that is already in the SDK to get a new instance of the config to pass to my other clients

[hasbro17]

@nicolaferraro Yes I think exposing the config should be alright since there's a legitimate use case.
Can you please update the CHANGELOG with:

The SDK exposes the Kubernetes rest config via `k8sclient.GetKubeConfig()` #338

[hasbro17]

@nicolaferraro After #340 you might want to rebase and change this to:

func GetKubeConfig() *rest.Config {
	once.Do(newSingletonFactory)
	return singletonFactory.kubeConfig
}

[nicolaferraro]

Thanks @hasbro17, I think it should be ok now.