Use back 'network' in 'shuffle' memory config option names

The issue is based on the discussion outcome on Dev ML.

As there is no strong consensus about using new 'shuffle' naming for memory config options. We decided to fallback to the the existing naming and use 'network' instead of 'shuffle'. We still keeping the new naming convention with prefix 'taskmanager.memory.*'.

Basically, we just need to rename 'taskmanager.memory.shuffle.' to 'taskmanager.memory.network.' as 'shuffle' naming has never been released.