Replace custom ProxyServer (kubecfg/kubectl -proxy) with httputil.ReverseProxy

Fixes #1149 - kubecfg proxy "411 Length Required" error on POST/PUT.
Yes, for instance `path.Join("a", "/b/")` would be `a/b` - notice missing slash at the end. There's probably no case where an API path ends with a `/` currently, but it's more future-proof that way. Plus, this function should be a bit faster than `path.Join`.