fix #3182 #3183: change backup retention mask logic

fix #3182 #3183: change backup retention mask logic

instead of relying on the contentTypeField (which does not need to
exists, e.g. for iscsi), explicitely write it into the panel/icon
mapping and check that

better would be if we query the backend about storage capabilities,
but such an api call does not exist yet, so this should be ok for now

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>