MONITOR: Enable an implicit files domain if one is not configured
    
    If SSSD is compiled with --enable-files-domain, the loading of the
    domains changes such that:
        * if no domain with id_provider=files exists in the config file, an
          implicit SSSD files domain is added
        * this domain is always first in the list
    
    The administrator is free to create a files domain in the config file
    himself and either place it at the end of the list or not enable it at
    all.
    
    Resolves:
    https://pagure.io/SSSD/sssd/issue/3112
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>