Cleanup coreos-oem-inject.py

The current script used to inject the IPA container into CoreOS is
not pep8 compliant, has requirements that are not in
global-requirements, and never has tests run. Now requirement on
plumbum is removed, script is pep8 compliant, flake8 runs against the
script when tests are run.

Closes-bug: #1337551

Change-Id: I14d3e46e60c4e072f34f80dc50d7214079953f49