Rename serializePropListQuantum -> serializePropList

The method serializes list of properties; 'quantum' is just confusing.
Move getObjectPropertyValue() call out of serializePropList()

Serialization method should do what it says and nothing more. It now
also doesn't fail.
Don't send undefined properties to initiator in getObjectPropListReq
Don't query Tracker for values it doesn't support
Add test case for FSStoragePlugin::getChildPropertyValues()