Add client side validation to kubecfg and kubectl.

This addresses at least part of #1352.  Builds on #2492.  Please only review the second commit.
Server should be able to turn off swagger (security risk?  Hosting choice?). Client should be able to work against a server where swagger is disabled (404 / 401 / others).  Is client validation that important beyond what we already have?

After all, we have mostly full server validation - what does this add on top?