Pk5/add image parsing

[patrykkulik-microsoft]

1. The _find_image_lines function now points at the template generated by the helm template command to find lines that contain images. We no longer need recursive search because all images are in the template. We also wanted to make the image list a Set() because images should be unique
2. Minor fix to how processors are accessed in the _validate_helm_template function
3. Two minor bug fixes in the artifact.py function to avoid infinite loop and trying to access undefined variables
4. Modified test_pre_validate_build.py because point (2) has broken the test
5. Added testing of the image finding, as well as some utility functions for unit tests
6. Added some input file templates for unit tests because without templating it is very difficult to make an OnboardingCNFCLIHandler that has the correct config.

[Cyclam]

Really good stuff. One minor possible refactor, and some generaly thoughts on unit tests, but even without those changes it's good to merge, so hitting approve. Up to you if you do want to make the changes.