CONFDB: Introduce SSSD domain type to distinguish POSIX and application domains
    
    Related to:
    https://pagure.io/SSSD/sssd/issue/3310
    
    Adds a new option that allows to distinguish domains that do contain
    POSIX users and groups and those that don't. The POSIX domains are the
    default. The non-POSIX domains are selected by selecting an
    "application" type domain.
    
    Reviewed-by: Sumit Bose <sbose@redhat.com>
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>