Fix default config behaviour

[fbeneventi]

Currently the normal behaviour is that the configuration file acts as a default and command line as an override.
Putting default here, override default file config option from config file and this is wrong.

https://github.com/fbeneventi/examon-common/blob/f20b896724edd15099559c12c79399c63551438d/examon/utils/config.py#L27-L31