Proposal: Merge jenkins jobs test.getDependency and systemtest.getDependency

[adamfarley]

Summary

This issue is to propose and discuss fusing jenkins jobs test.getDependency and systemtest.getDependency

Subtasks

• Add systemtest dependencies into the test.getDependency array and remove systemtest-specific logic from getDependencies.pl in TKG repo.
• Run the new test.getDependency job to populate the artifacts list.
• Remove all references to systemtest.getDependency
• Rename test.getDependency to test.getDependencies (as we are "getting" more than one dependency jar)
• Remove all references to the systemtest_prereqs folder (both for storage and retrieval)
• Consider if we should delete the systemtest.getDependency job (as doing so may break backwards compatibility).

Notes

Originally discussed here: #6753 (comment)