document arguments for --server-connect-attribute

[btm]

We should document the possible values you can give ---server-connect-attribute. It looks like these are values from the EC2 Instance Object. I'd guess the

• private_dns_name
• private_ip_address
• public_dns_name
• public_ip_address

These should be in the README, If we fix #344 we probably reduce the need for this to being pretty rare, but then we can change the description from:

The EC2 server attribute to use for SSH connection. Use this attr for creating VPC instances along with --associate-eip

To:
The EC2 server attribute to use for the SSH connection if necessary, e.g. public_ip_address or private_ip_address

We should print this somewhere to. If you or we get this wrong, you just hang on "Waiting for sshd access to become available" until you realize it (or don't!). I'd recommend we print the selection inside #ssh_connect_host, and print something that includes both the actual address and the attribute we chose:

"SSH Target Address: #{server.whatever} (public_ip_address)"