Store: Storage_maintenance always set as auto

Set auto maintenance delay as default. This allows to fallback to the auto mode, even if the overridden shell config points to an empty value.

In addition to that, it sets the config value to None, avoiding storing the default value (equivalent to the None fallback) into the config file.

Manually testing the MR

Check the maintenance status with a config where

"shell": {}

and look at the info events

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR