Empty string instead of None

Replace default values for all the fields that
are mandatory in the backend.

This could be checked at the backend also.

Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>
Add HarvestLogger

Add a separate and persistent, exclusively, for harvest
client activities.

Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>