Add recipe to create snapshots of EBS volumes.

The ebs_snapshots recipe will set up a snapshot script and schedule
it with cron to back up mounted EBS volumes. This recipe must come
after the ebs_data recipe in the run list.

Introduces attributes for setting the snapshot region and schedule.

Most of the heavy lifing is performed by the ec2-consistent-snapshot
script: <https://github.com/alestic/ec2-consistent-snapshot>.