adding support for AssociatePublicIpAddress for use with non-default VPCs

Hey Mitchell,

Pull request for the goamz changes for https://github.com/mitchellh/packer/issues/578 - can you skim this and apply it once your happy it looks good? I haven't been able to test it locally but it seems sane from a visual inspection here.

The test suite may need updating for it also at some point...

Thanks.