Add option to enable a simple CORS implementation for the api server

Related to https://github.com/GoogleCloudPlatform/kubernetes/issues/1128
@jwforres I've just realized this won't work when apiserver is reverse-proxied by Nginx + Basic Auth.
Preflight request (OPTIONS) does not include credentials, Basic Auth in this case. So, OPTIONS request will never reach apiserver as Nginx will respond with 401 authentication required error.