Uniformize handling of -server_version flag of kubecfg to match -version

In particular, add support for -server_version=raw and use matching
format for the output of -version and -server_version.

The "normal" format is essentially defined by (version.Info) String()
method, so future updates to that method will be reflected on both.

Full version information is still available by using the "raw" flag.

Fixes: #1092