Use private IPv4 address instead of an Elastic IP about ec2-boot-bench

[cperciva]

Good idea. I wasn't doing this because I was doing my initial testing from outside of EC2, but you're absolutely right that if we're testing from inside EC2 it makes sense to use the private address instead.

If you look at 96b2e67 you'll see roughly what's necessary:

1. Add a variable to record if we're using this option.
2. Add a GETOPT_OPT which sets the variable to 1.
3. Add it to usage().
4. Change the IPv4 address extraction code to look for "privateIpAddress" if that variable is set.

If you're not comfortable writing the patch (I totally understand "I don't use this language often"! I say that a lot with Python code) I can put it together once I catch up with some other re:Invent-related work.