Jenkins Slave Jinja Template should ONLY override Dockerfile FROM when BUILDER_IMAGE_NAME is defined.

[InfoSec812]

For older releases of Labs CI/CD, the BUILDER_IMAGE_NAME value is not set in the inventory, so the current Jinja template will cause the Maven slave to build without Maven installed because it defaults to a base image of quay.io/openshift/origin-jenkins-agent-base:4.2. Change the Jinja template HERE to use a conditional block instead of the defaulting of values.