Use clientcmd.NewDefaultClientConfigLoadingRules()
to search the standard locations for existing KubeConfig
files if the user did not provide a --kubeconfig flag.

If the user provides a --kubeconfig flag respect it.

Also do the following:
- Add a unit test in token_test.go for the
NewDefaultClientConfigLoadingRules() functionality.
- Rename the TestConfig and related constants to make
them unique to this file.

Fixes kubernetes/kubeadm#198