Unskip Integ test `test_hp_contract_mpi_script` for SageMaker Train

Integ test `test_hp_contract_mpi_script` in [test_model_trainer.py](https://github.com/aws/sagemaker-python-sdk/blob/master/sagemaker-train/tests/integ/train/test_model_trainer.py) was skipped for some reson, we need to unskip it since it is critical test. While there are some bugs/config issues in the test, we need to fix it until it get passed.