StorageObj nested objects names and creation

[polsm91]

Nested IStorage weren't created properly in the StorageObj. The name given to nested IStorage objects was wrong. Sometimes accessing the nested IStorage objects created a new empty object instead of accessing the stored data in Cassandra.

Fixed by:

• The init method properly creates the nested IStorage objects.
• Doing a getattr retrieves the nested IStorage that existed when the make persistent was called. If setattr was called, returns the last IStorage object assigned.
• Calling a setattr of a persistent StorageObj makes persistent the given object.
• Calling make _persistent on a StorageObj, accesses the hecuba.istorage table to retrieve the metadata (tokens and such)
• Added tests to verify the expected behaviour

Fixes #176 for StorageObjects