code: def devices(self):
          
          install_devices = self.install_devices
          if 'bootstrap-system-devices' in env.instance.config:
              devices = set(env.instance.config['bootstrap-system-devices'].split())
          else:
              devices = set(self.sysctl_devices)
              for sysctl_device in self.sysctl_devices:
                  for install_device in install_devices:
                      if install_device.startswith(sysctl_device):
                          devices.remove(sysctl_device)
          return devices
docs: computes the name of the disk devices that are suitable
          installation targets by subtracting CDROM- and USB devices
          from the list of total mounts.