Add the possibility to specify the log file in the configuration file

There are two options that we add support for here:
- log_file: specifies the file where to log the actions performed by
  repo_manager
- unique_log: a boolean specifying if there should one log file or if
  the action should be logged in the specified file in addition to the
  default log
Expand the sample configuration file to include the two new options log_file and unique_log
Prevent having two handlers logging in the same file