Add p1-keycloak-plugin:3.6.7 image reference to tests/images.txt

Feature Request

Why

Now that the methodology for generating the images.txt involves parsing config files, we need to make sure that all required images are included.

Proposed Solution

Add registry1.dso.mil/ironbank/big-bang/p1-keycloak-plugin:3.6.7 to tests/images.txt