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 was thinking of merging them. Given that it is a set and that amazon provides the equals and hashcode just adding all to the set should be fine? Right if there are any on the image they will have precedence. That might not be what you want. Maybe there is always on the ami?

So I see three options.
1. Leave as is
2. Merge config and AMI (I might actually prefer this)
3. Give precedence to config.