Add option to use the private IP in a VPC for ssh

This allows you to connect to the instances when the security group only
allows access via the private IP over a VPCs VPN connection.

Note: it has to be set as a symbol and not a string in the
with_fog_ec2_provisioner options, otherwise the option isn't recognized.

I've updated the example config.json file and README to include this option (with a value of false, but showing people that it's there in case it's needed), and if it's left out of the config, it defaults to false also.