consolidate logging in object_storage package

[sleerssen]

There are (at least) three logging instances being created in the object_storage package, and that makes it a bit messy for callers to redirect logging to another logging instance owned by the caller. It would be nicer if object_storage.__init__.py created a singleton that the rest of the library used. Then replacing the logging instance would be as simple as reassigning the top level logger with the callers logger.