cielito.system.swap role for a VM

Closes proxmox#17, where requirement is stated. We deal with it here because it's not a proxmox specific issue.

Recent cloud images do no longer have a swap. As it is needed in several cases, let's code a role to generate a swap, in a file or in an existing disk partition.

Note: this role is intended to full virtual machines, in containers swap is provided from the contenerized system.