Update documentation on how to retire a host - WIP

 1. long before (weeks or months) the machine is decomissioned, make

    sure users are aware it will go away and of its replacement services

 2. if applicable, stop the VM: `virsh shutdown $host`, or at least

    stop the primary service on the machine

 3. if applicable, undefine the VM: `virsh undefine $host`

 2. if applicable, stop the VM:

    * If the VM is on a KVM host: `virsh shutdown $host`, or at least stop the

    primary service on the machine

    * If the machine is on ganeti: gnt-instance remove $host

 3. On KVM hosts, undefine the VM: `virsh undefine $host`

 4. wipe host data, possibly with a delay:

    * if applicable, remove the LVM logical volumes or virtual disk

      files:

    * On KVM hosts, remove the LVM logical volumes or virtual disk files:

        `lvs` will list the logical volumes on the machine

          echo 'lvremove -y vgname/lvname' | at now + 7 days

        `echo 'lvremove -y vgname/lvname' | at now + 7 days`

    * for a normal machine or a machine we do not own the parent host

      for, wipe the disks using the method described below