Add unittests for dss_host.py and dss_target.py #71

Job	Run time
lint (shellcheck) / build-aws	5m 8s
build (dss-sdk-unit-test) / build-aws	9m 40s
lint (pycodestyle) / build-aws	6m 36s
build (dss-sdk-build) / build-aws	9m 12s
scan / build-aws	9m 11s
	39m 47s

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