fleetctl: hyphens in TLS flag names

fleetctl's TLS flags are `--cafile`, `--keyfile` and `--certfile`. etcdctl uses `--ca-file`, `--key-file` and `--cert-file`. curl's equivalent flags are `--cacert`, `--key` and `--cert`. Some level of consistency would be nice here. I'm not really excited about curl's flags (`fleetctl --key=foo` isn't really descriptive),  so I'm proposing we throw a hyphen in there.