fix(contrib): check that coreos/user-data has a discovery_url set

It is all-too-easy to forget to modify `contrib/coreos/user-data` to include a new etcd discovery URL. This adds a validation script `contrib/util/check-user-data.sh` that parses the user-data file as YAML and errors out if it doesn't find a `coreos_etcd_discovery` entry. This check is skipped if `DEIS_NUM_INSTANCES` is 1.

(This is the kind of logic that will be much more maintainable when added to a `deisctl` command-line tool. See #1018.)