Add backup/restore testing documentation, and implement few possible automated backup/restore testing scenarios

[yaroslav-gwit]

Some backup and virtualisation systems out there have these nice options for automated backup/restore testing, that include but not limited to:

• Taking a VNC screenshot and running OCR on it to check if certain text exists, or emailing that screenshot to the user
• Resetting network configuration and pinging the VM in order to check if it comes back
• Using Cloud Init to run a certain script on boot (the script itself can be sending an email from within the VM, reaching out to a webhook, etc)

With all that said, I would like to implement and document some of the more basic options for the automated backup/restore testing within Hoster too. For now it will be a simple ping check, but if there is more interest in this feature - I'll implement some more advanced things too.