def load_merge_candidate(self, filename=None, config=None):
raise NotImplementedError<EOL>
Populates the candidate configuration. You can populate it from a file or from a string.
If you send both a filename and a string containing the configuration, the file takes
precedence.

If you use this method the existing configuration will be merged with the candidate
configuration once you commit the changes. This method will not change the configuration
by itself.

:param filename: Path to the file containing the desired configuration. By default is None.
:param config: String containing the desired configuration.
:raise MergeConfigException: If there is an error on the configuration sent.