A silence should be added for backups on newly-provisioned machines

With the new monitoring system, we have a value of 0 for the timestamp of the last backup until the first full backup of a machine is run.

To avoid useless alerts right after provisioning a new host, we should modify our fabric scripts to trigger the addition of a silence for the backup alerts of that host (e.g. alias=$fqdn and job=bacula).

The expiration delay that we need is not super clear to me. Is 2 days about right?