[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.

ec2/spark_ec2.py

@@ -1242,6 +1242,10 @@ def get_ip_address(instance, private_ips=False):
 def get_dns_name(instance, private_ips=False):
     dns = instance.public_dns_name if not private_ips else \
         instance.private_ip_address
+    if not dns:
+        raise UsageError("Failed to determine hostname of {0}.\n"
+                         "Please check that you provided --private-ips if "
+                         "necessary".format(instance))
     return dns