Move common live functionality to a new layer

os/readme.md

@@ -93,9 +93,10 @@ Here is a list of features.
   - timezone
   - list of locales
   - bootloader background image
-- `training-live`
+- `live` (all live variants)
   - disable lock on blank screen
   - disable software update notifications
+- `training-live`
   - automatic login
   - sudo without password
 - `training-installer`
@@ -111,8 +112,6 @@ Here is a list of features.
 - `contestant`
   - disable bluetooth
   - disable sleep
-  - disable lock on blank screen
-  - disable software update notifications
   - disable some panels in gnome-control-center
   - disable automatic mounting of storage media
   - polkit rules which block changing network settings and mounting storage media (it prompts for the root password)