Add support for block device mappings in AWS.

This adds support for AWS block device mappings (http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/block-device-mapping-concepts.html)

This requires libcloud >= 0.12.1 (http://svn.apache.org/viewvc/libcloud/trunk/libcloud/compute/drivers/ec2.py?r1=1438350&r2=1438349&pathrev=1438350)