IPA_COMMON: Introduce ipa_get_host_attrs()
    
    By adding this method it can reused in the future for new backend
    modules.
    
    Related:
    https://pagure.io/SSSD/sssd/issue/2995
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>