Prepare Automated Deployment fails when the VPC has no defined purpose.

[grahamc]

P.A.D. requires that the VPC be configured with the immutable_metadata tag, as described here: https://github.com/Netflix/asgard/wiki/VPC-Configuration. Currently there is no messaging around this being the case when faced with the stack trace, perhaps a blank page when clicking the button, or the scroll bar stuck at 50% without any progress.