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.
Merge pull request #28 from mivok/mh/private_ip_ssh

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