Properly handle loading KubeConfig certs/keys

[danopia]

CA and key data can be either in-line or in a file, and there's currently no abstraction over this disparity. There's technically some complexity around file paths (mostly relative paths - they're supposed to be relative to the file that the path was originally found in).

The KubeConfigContext class should expose a way to reliably grab the cluster CA and the client key&cert, taking into account where the relevant file paths came from.