Skeleton directories

[mikery]

Placeholder for a feature I need to add.

When a snapshot is not found (i.e. first time an instance is booted), a local directory on an already-mounted file system should be used to populate the new volume, rather than formatting it and leaving it empty.

• create the files in the skeleton dir, build AMI
• launch the instance
• no snapshot will be found, so data will be copied from skeleton dir
• < backup is taken, machine is rebooted >
• now that snapshots exist, they are used instead of the skeleton data

Changes:

• add a skeleton_dir attribute to volume class
• cp contents of this dir to volume mount location