Require config file for staring Elasticsearch

This should help prevent Elasticsearch from coming up with garbage config.
It isn't so bad if it comes up with default config files because it won't
join the cluster and cause it trouble but if it comes up with half the
config files changed then it'll join the cluster without enough heap space
and cause havoc.  This should prevent that, if not the service starting
with default configuration.

Change-Id: I03f260440b06078d7d6c51758297dcd94f1e6d2d