change backup disk option to boolean

[tnyeanderson]

In the upstream docs the backup option is a boolean.

See reasoning in Telmate/proxmox-api-go#231 for why the default value is true. That PR and this one should finally allow backups to be disabled.

Fixes: #429

[tnyeanderson]

Looks like the container side is already correct.

By default additional mount points besides the Root Disk mount point are not included in backups. For volume mount points you can set the Backup option to include the mount point in the backup. Device and bind mounts are never backed up as their content is managed outside the Proxmox VE storage library.

This has been tested with the PR from proxmox-api-go and should be ready to merge.