fix test_get_yarn_spark_resource_config after AWS_REGION was added into properties

[pavel-balas]

Issue #, if available:

The pull/75 change introduced AWS_REGION variables, but they are not reflected in unit test

% pipenv run python -m pytest -s -vv test/unit
...
E         Left contains 2 more items:
E         {'spark.executorEnv.AWS_REGION': 'None',
E          'spark.yarn.appMasterEnv.AWS_REGION': 'None'}
...
FAILED test/unit/test_bootstrapper.py::test_get_yarn_spark_resource_config - assert {'spark.default.parallelism': '8',\n 'spark.driver.defaultJavaOptions': "-XX:OnOutOfMemoryError='kill -9 %p' "\n          ...

Description of changes:

I don't like just update assertion to have string "None", so I'm patching the os variable to have a reasonable value and then asserting that the value made it to the spark properties.

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