Create log_file_depot using relationships

https://bugzilla.redhat.com/show_bug.cgi?id=965889
Simplify all the things

https://bugzilla.redhat.com/show_bug.cgi?id=965889
Ensure that all FileDepots are subclassed

https://bugzilla.redhat.com/show_bug.cgi?id=965889
Simply state that we are checking whether credentials are required

https://bugzilla.redhat.com/show_bug.cgi?id=965889
Fix validations for MiqSchedule backup depots

https://bugzilla.redhat.com/show_bug.cgi?id=965889
Preload subclasses of FileDepot so that they appear in supported_depots

https://bugzilla.redhat.com/show_bug.cgi?id=965889