Add the ability to specify KUBECONFIG variable in openebsctl

[smijolovic]

Currently openebsctl looks in the users directory for .kube/config file, and when it does not exist exits with the following error:
failed to build Kubernetes clientset: Could not build config from flags: stat /home/chaasmadmin/.kube/config: no such file or directory

This feature request is to add a KUBECONFIG variable setting for openebsctl to point to the location of the kubectl config file.