software/fluentd: limit buffer file max size

if chunk size is too large, wendelin can't handle it

Traceback (most recent call last):
  File "erp5/component/document/erp5_version.py", line 80, in ingest
    self.REQUEST.form['data_chunk'] = self.REQUEST.get('BODY')
  File "ZPublisher/HTTPRequest.py", line 1058, in get
    v = self.other[key] = self._fs.value
  File "ZPublisher/HTTPRequest.py", line 1371, in __get__
    raise BadRequest("data exceeds memory limit")
zExceptions.BadRequest: data exceeds memory limit