Add more parameters to utils.WriteFile

- Make closing file optional: Required by ganeti-watcher to keep
  file open after writing it. Changes return value of utils.WriteFile
  if "close" parameter evaluates to True.
- Pre- and post-write functions: Can be used to lock files. This
  will be used by ganeti-watcher to lock the temporary file before
  renaming.

Reviewed-by: iustinp