Trades runtime complexity for spacial complexity when modifying
large amounts of contexts on a kubeconfig.
In cases where there are few destination filenames for a given
amount of contexts, but a large amount of contexts, this patch
prevents reading and writing to the same file (or small number
of files) over and over again needlessly.
Kubernetes-commit: d5651948cf1a14ed284b4708e2057e4cbc72bcbe
Before this change, if the config was empty, ConfirmUsable() would
return an "invalid configuration" error instead of examining and
honoring the value of the --namespace flag. This change looks at the
overrides first, and returns the overridden value if it exists before
attempting to check if the config is usable. This is most applicable to
in-cluster clients, where they don't have a kubeconfig but they do have
a token and can use KUBERNETES_SERVICE_HOST/_PORT.
Kubernetes-commit: 23e32b100fb9745b70203c30716697bd03926313
