Suggested fix for issue #334

In order to support automatically taking any block device mapping of an AMI into an ASG they can be taken from the image itself. Any default EBS volumes on the AMI will be mounted as usual. 

This solves the issue we've been having with asgard not mounting the ephemeral disks in the AMI
I think it's a mixture of number 2 and 3 ... They should definitely be merged, allowing the configuration to override image mappings. What do you think about that?