consider do not allow to create stacked KUBECONFIG & check if KUBECONFIG exist

[Dentrax]

Similar to issue #599, I think we should not allow to the creation of stacked KUBECONFIGs. As @morlay pointed out the root cause of that issue here, my $KUBECONFIG by default looks like as follows:

$ echo $KUBECONFIG
:/Users/furkan.turkal/.kube/config:/Users/furkan.turkal/.kube/foo:/Users/furkan.turkal/.kube/bar:/Users/furkan.turkal/.kube/baz

We can fix the issue here by checking the given KUBECONFIG whether exist just before we push Endpoint into NodeGroup list.

[morlay]

@Dentrax I prefer to support to load Colon-separated KUBECONFIG

[Dentrax]

Fixed by #761