Improve handling of contexts with no configured namespace

Improve handling of contexts with no configured namespace

* kubed.el (kubed--namespace): Fallback to prompting when no
default namespace is available.
(kubed-default-context-and-namespace): Prompt for default
namespace when there is none.
(kubed-current-namespace): Clarify that this function may
return nil.
(kubed-read-resource-name): Make CONTEXT argument
non-optional.
(kubed-read-resource-type): Update accordingly.