configurize possible postgres extensions

Change-Id: I57bf7eebf4a9f5a828dffb3d4b689b6cac9aa0f0
Reviewed-on: https://gerrit.instructure.com/40197
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>