Fix migration for openldap DS
    
    openldap server does not store its schema in cn=schema entry, but
    rather in cn=subschema. Add a fallback to ldap2 plugin to read from
    this entry when cn=schema is not found. ldap2 plugin uses the schema
    when doing some of the automatic encoding, like an automatic
    encoding of DN object.
    
    IPA migration plugin DN attribute processing is now also more
    tolerant when it finds that some DN attribute was not autoencoded.
    It tries to convert it to DN on its own and report a warning and
    continue with user processing when the conversion fails instead of
    crashing with AssertionError and thus abandoning the whole
    migration run.
    
    https://fedorahosted.org/freeipa/ticket/3372