Refactor: python: Improve startonebyone.py.

• Disable pylint warnings we can't really do anything about.

• Get rid of the unused ns variable. The ClusterManager object cares about NodeStatus, but this individual test does not. This seems like a leftover from a previous refactoring.

• Rename stopall to _stopall. This is a private attribute, so name it like one.

• Simplify how the failed list is checked.