* [log_format] Add an option to change the log format

Hacing an option to change the log format could be useful when logs
are shipped into an aggregator such as Elasticsearch.

The current format is very hard to parse.

* [log_format] Set default Formatter

Because mycroft.configuration.Configuration() class import LOG class,
a default Formatter have to be defined before import the
mycroft.configuration.Configuration() class.