Use public DNS hostname everywhere in spark_ec2.py

The spark_ec2.py script uses public_dns_name everywhere in the script except for testing ssh availability, which is done using the public ip address of the instances. This breaks the script for users who are deploying the cluster with a private-network-only security group. The fix is to use public_dns_name in the remaining place.

I am submitting a pull-request alongside this bug report.