Use common base URL for KMT images

[gjulianm]

What does this PR do?

Fixes how images are defined in the platforms.json file and how they are downloaded. This fixes inconsistencies in how the images were downloaded (for example, sometimes the branch name would be ignored and replaced with master).

Motivation

Consistency in how images are referred to and downloaded.

Additional Notes

Possible Drawbacks / Trade-offs

Describe how to test/QA your changes