KRB5: Offline operation with disabled domain
    
    https://fedorahosted.org/sssd/ticket/2637
    
    If a subdomain is in the disabled state, switch krb5_child operation
    into offline mode.
    
    Similarly, instead of marking the whole back end as offline, mark just
    the domain as offline -- depending on the domain type, this would mark
    the whole back end or just inactivate subdomain.
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>