For file discovery, in case the user feeds a file for the CA
from the kubeconfig, make sure it's preloaded and embedded using
the new function EnsureCertificateAuthorityIsEmbedded().
This commit also applies cleanup:
- unroll validateKubeConfig() into ValidateConfigInfo() as this way
the default cluster can be re-used.
- in ValidateConfigInfo() reuse the variable config instead of creating
a new variable kubeconfig.
- make the Ensure* functions return descriptive errors instead of
wrapping the errors on the side of the callers.
540c272f7b