Possibility to use additional ReportStore

This PR gives the possibilty to use new subclasses of ReportStore.

Currently, we have only two choices : localfs and s3. With this PR, we can specify in the settingstorage_type the string name of the subclass.

You can find an example of a new subclass of ReportStore in the following link : https://github.com/ADubus/fun-apps/blob/generic_reportstore/report_grades/reportstore.py This subclass is useful for the platforms that don't use Amazon S3