Adding new test for idempotent changes to HEC configuration

[nwang92]

When tested with the splunk-ansible branch here, I get a success:

(venv) nwang@nwang-MBP-CCEA8:~/github/docker-splunk (develop) $ pytest -sv tests/test_docker_splunk.py::TestDockerSplunk::test_adhoc_1so_hec_idempotence --platform redhat-8
======== test session starts ========
platform darwin -- Python 2.7.16, pytest-4.6.9, py-1.8.1, pluggy-0.13.1 -- /Users/nwang/github/docker-splunk/venv/bin/python
cachedir: .pytest_cache
rootdir: /Users/nwang/github/docker-splunk/tests, inifile: pytest.ini
collected 1 item

tests/test_docker_splunk.py::TestDockerSplunk::test_adhoc_1so_hec_idempotence PASSED

======== warnings summary ========
venv/lib/python2.7/site-packages/_pytest/mark/structures.py:335
  /Users/nwang/github/docker-splunk/venv/lib/python2.7/site-packages/_pytest/mark/structures.py:335: PytestUnknownMarkWarning: Unknown pytest.mark.large - is this a typo?  You can register custom marks to avoid this warning - for details, see https://docs.pytest.org/en/latest/mark.html
    PytestUnknownMarkWarning,

-- Docs: https://docs.pytest.org/en/latest/warnings.html
======== 1 passed, 1 warnings in 143.64 seconds ========

I'll also fix some of the other HEC tests that fail once the splunk-ansible PR is merged.