Adding test for upgrades from older splunk images

[nwang92]

$ pytest -sv tests/test_docker_splunk.py::TestDockerSplunk::test_adhoc_1so_upgrade
======================= test session starts =======================
platform darwin -- Python 2.7.15, pytest-4.0.2, py-1.7.0, pluggy-0.8.0 -- /Users/nwang/github/docker-splunk/venv/bin/python2.7
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_upgrade PASSED

======================= 1 passed in 62.76 seconds =======================