erp5_web_service: add also prefix when writing temporary file

In practice, SFTP-based client processes often use a prefix based
filter when retrieving files on the server.

ie: there is one SFTP server between 2 systems, used for different
exchanges. The SFTP client will do something like (found in the wild):
process1(file_list=sftp.list(path, prefix='PROCESS_1'))
process2(file_list=sftp.list(path, prefix='PROCESS_2'))

Our implementation of temporary_file_on_write only sets a suffix,
which doesn't work for the previous SFTP client. So let's add also a
prefix when writing temporary files.

bt5/erp5_web_service/DocumentTemplateItem/portal_components/document.erp5.FTPConnector.py

@@ -105,8 +105,8 @@ class FTPConnector(XMLObject):
     try:
       if self.isUseTemporaryFileOnWrite():
         # Simulation transaction system
-        conn.writeFile(remotepath, '%s.tmp' % filename, data, confirm=confirm)
-        self.activate(activity='SQLQueue').renameFile('%s/%s.tmp' % (remotepath, filename),
+        conn.writeFile(remotepath, '_%s.tmp' % filename, data, confirm=confirm)
+        self.activate(activity='SQLQueue').renameFile('_%s/%s.tmp' % (remotepath, filename),
                                                       '%s/%s' % (remotepath, filename))
       else:
         conn.writeFile(remotepath, '%s' % filename, data, confirm=confirm)