Update the format of saved_attributes configuration

Holger wanted a template for the config where motors do not need to be copy-pasted for each group. The trade-off is that all groups must now have the same motors, but that's the current use case.

The 'groups' key can be omitted in the config (Holger wanted to remove all groups info from the config, but I'm pretty sure some use case will arise), then 'Default' will be used.