With login node flag

This pull request adds the ability to override the login node's public IP address with a user-supplied value. The main use case for this is e-mails on the community list where users want to run in CloudStack but don't want to set up port forwarding from the outside world to their head node, and Eucalyptus, where private DNS addresses don't resolve.

To work around the above problems, this pull lets users run a bunch of machines as a virtualized cluster, specify their private IPs in their ips.yaml, and specify `--login_host` as the public FQDN/IP address of the `login` machine. Since `login_host` is publicly resolvable, this gets around port forwarding for CloudStack, and since private IPs are used (and not public DNS), this gets around that problem in Eucalyptus.

Tested on the Eucalyptus Community Cloud, and works!