Fix registry upload purging configuration

!3379 (merged) added the registry uploadpurging config option, but it put the variable in the wrong place.

As documented in https://docs.docker.com/v17.09/registry/configuration/, the correct configuration is to put the maintenance section under the storage header:

storage:
  ...
  maintenance:
    uploadpurging:
      enabled: true
      age: 168h
      interval: 24h
      dryrun: false

However, the current config produces the following format:

storage: ...
maintenance:
  uploadpurging:
    enabled: false