BE: Pass in attribute to look up with instead of hardcoding SYSDB_NAME
    
    In later patches, we will implement refreshes for AD or IPA which might
    refresh objects that do not have a name yet, but always do have a different
    attribute, like a SID or a uniqueID. In this case, it's better to use that
    different attribute instead of name.
    
    This patch allows the caller to tell the refresh module which attribute
    to use.
    
    Related:
    https://pagure.io/SSSD/sssd/issue/4012
    
    Reviewed-by: Sumit Bose <sbose@redhat.com>
    (cherry picked from commit d1eb0a70de3c98ca9dc03a0b79287f4ce6ee4855)
    
    Reviewed-by: Sumit Bose <sbose@redhat.com>