[SPARK-11991] fixes

If `--private-ips` is required but not provided, spark_ec2.py may behave inappropriately, including attempting to ssh to localhost in attempts to verify ssh connectivity to the cluster.

This fixes that behavior by raising a `UsageError` exception if `get_dns_name` is unable to determine a hostname as a result.

Author: Jeremy Derr <jcderr@radius.com>

Closes #9975 from jcderr/SPARK-11991/ec_spark.py_hostname_check.