This changes clientcmd to skip the default cluster, but preserves the
behavior in kubectl. This prevents the possibility of an administrator
misconfiguration in kubelet or other server component from allowing a
third party who can bind to 8080 on that host from potentially
impersonating an API server and gaining root access.
Before this change, Authorize() method was just returning an error,
regardless of whether the user is unauthorized or whether there
is some other unrelated error. Returning boolean with information
about user authorization and error (which should be unrelated to
the authorization) separately will make it easier to debug.
Fixes#27974
