Backporting to 1.2-2 version of the container - store ground truth and predictions on validation folds to output folder

[iaroslav-ai]

Description of changes:
Backporting functionality to store predictions from xgboost container to 1.2-2 version of xgboost container.
See original PR for more details: #230 .

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[iaroslav-ai]

I am able to build the image on SageMaker notebook with following commands:

docker build -t xgboost-container-base:1.2-2-cpu-py3 -f docker/1.2-2/base/Dockerfile.cpu .
python setup.py bdist_wheel --universal
docker build -t preprod-xgboost-container:1.2-2-cpu-py3 -f docker/1.2-2/final/Dockerfile.cpu .

when running integration tests, they are passing:

pytest test/integration/local --docker-base-name preprod-xgboost-container \
>                   --tag 1.2-2-cpu-py3 \
>                   --py-version 3 \
>                   --framework-version 1.2-2
=================================================== test session starts ===================================================
platform linux -- Python 3.7.10, pytest-6.2.2, py-1.10.0, pluggy-0.13.1
rootdir: /home/ec2-user/SageMaker/sagemaker-xgboost-container
plugins: anyio-3.3.2
collected 23 items

test/integration/local/test_abalone.py .....                                                                        [ 21%]
test/integration/local/test_boston.py .                                                                             [ 26%]
test/integration/local/test_early_stopping.py ....                                                                  [ 43%]
test/integration/local/test_kfold.py .....                                                                          [ 65%]
test/integration/local/test_multiple_model_endpoint.py .......                                                      [ 95%]
test/integration/local/test_versions.py .                                                                           [100%]

==================================================== warnings summary =====================================================
../../anaconda3/lib/python3.7/site-packages/sagemaker_containers/_mapping.py:151
  /home/ec2-user/anaconda3/lib/python3.7/site-packages/sagemaker_containers/_mapping.py:151: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated since Python 3.3,and in 3.9 it will stop working
    class MappingMixin(collections.Mapping):

../../anaconda3/lib/python3.7/site-packages/pkg_resources/__init__.py:1136
  /home/ec2-user/anaconda3/lib/python3.7/site-packages/pkg_resources/__init__.py:1136: DeprecationWarning: Use of .. or absolute path in a resource path is not allowed and will raise exceptions in a future release.
    self, resource_name

-- Docs: https://docs.pytest.org/en/stable/warnings.html
======================================= 23 passed, 2 warnings in 179.62s (0:02:59) ========================================