ASGARD-1124 - Single EBS volume for m3 instances

Change Asgard's m3.* launch behavior to add one block device mapping
/dev/sdb instead of the current four block device mappings.
Merge pull request #300 from joesondow/ASGARD-1124-single-EBS-volume-for-m3-instances

ASGARD-1124 - Single EBS volume for m3 instances