Upgraded to use SageMaker python lib v2.x

• All the exercises were updated to use SageMaker Python Library v2.x;
• The warm up exercise was divided in four distinct notebooks;
• The warm up exercise was simplified, specially the Part 1/4;
• Part 4/4 is related to model monitor and now you can kick off a processing job manually and don't wait for the scheduler (each 1h);
• Part 2 of the workshop is optional. Now you don't need to create a custom container to train a model. You can use the built-in XGBoost;
• Part 3 supports both XGBoost built-in or RandomForest custom container (controlled by a boolean var);
• The cloudformation now launches a Notebook Instance with the Python3 Kernel updated to SageMaker Python Library v2.x.