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.

Tested:
- Used cluster/kubecfg.sh to query local build and the server.

    $ cluster/kubecfg.sh -version
    Kubernetes version 0.2+, build 9316edfc0d2b28923fbb6eafa38458350859f926
    $ cluster/kubecfg.sh -server_version
    Server: Kubernetes version 0.2, build a0abb3815755d6a77eed2d07bb0aa7d255e4e769
    $ cluster/kubecfg.sh -version=raw
    version.Info{Major:"0", Minor:"2+", GitVersion:"v0.2-25-g9316edfc0d2b28", GitCommit:"9316edfc0d2b28923fbb6eafa38458350859f926", GitTreeState:"clean"}
    $ cluster/kubecfg.sh -server_version=raw
    version.Info{Major:"0", Minor:"2", GitVersion:"v0.2", GitCommit:"a0abb3815755d6a77eed2d07bb0aa7d255e4e769", GitTreeState:"clean"}

Fixes: #1092

Signed-off-by: Filipe Brandenburger <filbranden@google.com>
Merge pull request #1245 from filbranden/server_version1

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