add more checks to data structure

In manifests backups.pp and forgets.pp, before hashs $backups and $forgets are used by backup.pp and forget.pp, it is needed to add check about hash structure.

For instance, if we use the following hash, it must return an error :

backups:
  herealabel:
    repository_url: '/var/resticrepo'
    repository_password: blah
    at:
      frequency: 'weekly'
      excludes:
        - '/var/resticresticrepo'
        - '/sys'
        - '/proc'
        - '/mnt'

In this example, the key excludes is not at the right place in the hash.