[SPARK-5242]: Add --private-ips flag to EC2 script

The `spark_ec2.py` script currently references the `ip_address` and `public_dns_name` attributes of an instance. On private networks, these fields aren't set, so we have problems.

This PR introduces a `--private-ips` flag that instead refers to the `private_ip_address` attribute in both cases.

Author: Michelangelo D'Agostino <mdagostino@civisanalytics.com>

Closes #5244 from mdagost/ec2_private_nets and squashes the following commits:

b684c67 [Michelangelo D'Agostino] STY: A few python lint changes.
a4a2eac [Michelangelo D'Agostino] ENH: Fix IP's typo and refactor conditional logic into functions.
c004604 [Michelangelo D'Agostino] ENH: Add --private-ips flag.