Support for CoreUpdate options

This provides the [options for CoreUpdate](https://coreos.com/docs/coreupdate/coreos/coreupdate-configure-machines/).

Follow on PR to coreos-cluster-cli will come after this is merged and released.
Should check for `options.update` before... i.e. `if (options.update && options.update.group) {`. If you don't pre-check `options.update` you could get a `cannot read property server of undefined` error.

Also a style nit: `if (` with a space before the open parenthesis.