Fix broken integration tests

[gbmeuk]

tests are failing because ps is missing from the node base Docker image

Thanks for contributing to ContainerPilot! Please include with your pull request:

• A description of what you did for the changelog
  fix broken integration tests
• An explanation of why ContainerPilot needs this change
  so changes are properly tested
• How to verify that it works (most PRs need tests!)
  tests which were previously failing due to ps missing, no longer fails
• A link to the GitHub issue that it addresses

If you're contributing a new feature, it's usually better to open an issue to discuss the feature rather than open a pull request unless the feature is trivial.