migrate-ds: workaround to detect compat tree
    
    Migrate-ds needs to check if compat tree is enabled before
    migrating users and groups. The check is doing a base
    search on cn=compat,$SUFFIX and considers the compat tree
    enabled when the entry exists.
    
    Due to a bug in slapi-nis, the base search may return NotFound
    even though the compat tree is enabled. The workaround is to
    perform a base search on cn=users,cn=compat,$SUFFIX instead.
    
    Fixes: https://pagure.io/freeipa/issue/8984
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>