otptoken: fix import of DN
    
    Import DN from ipapython.dn rather than ipalib.plugins.baseldap.
    
    This will make it possible to move otptoken_sync to ipaclient.
    
    https://fedorahosted.org/freeipa/ticket/4739
    
    Reviewed-By: David Kupka <dkupka@redhat.com>