SmallStack: Implement automated backups for VMs

[rgooch]

Using vm-control a VM owner should be able to set up an automated backup schedule. As described in issue #468, the VM metadata and volume data will be saved to a remote storage system (i.e. AWS S3).
The Fleet Manager will orchestrate the automated backups, allowing for global rate limits to be applied.

The Fleet Manager will provide the Hypervisors with temporary credentials to give write access to the S3 bucket.