kubectl: add less verbose version

The kubectl version output is very complex and makes it hard for users
and vendors to give actionable information. For example during the
recent Kubernetes 1.4.3 TLS security scramble I had to write a one-liner
for users to get out the version number to give to figure out if they
are vulnerable:

    $ kubectl version | grep -i Server | sed  -n 's%.*GitVersion:"\([^"]*\).*%\1%p'

Instead this patch outputs simply output with `--short`

    ./kubectl version --short
    Client Version: v1.4.3
    Server Version: v1.4.3

kubectl-conventions.md

@@ -151,6 +151,9 @@ generation, etc., and display the output
 
 * `--output-version=...`: Convert the output to a different API group/version
 
+* `--short`: Output a compact summary of normal output; the format is subject
+to change and is optimizied for reading not parsing.
+
 * `--validate`: Validate the resource schema
 
 ## Output conventions