Add FileDepot.supported_protocols

https://bugzilla.redhat.com/show_bug.cgi?id=1173213
@matthewd @brandondunne 
We'd have to check the callers but I don't see why not.  In some cases we used class strings instead of the class itself to prevent requiring the world (a class that requires a class, that requires a class, etc.).  But, we aren't doing that here, since we have the klass required and call klass.name.