feat: add test for training operator in airgapped

[NohaIhab]

closes #919, see the issue for more details.

Summary

• adds a training job example to test in airgapped
• adds README.md for training operator test with testing instructions
• modifies the get-all-images.sh script to include the training job example image

[mvlassis]

Just tested in an airgapped environment and the tests work and pass! LGTM.