add common cartridge 1.3 variant support

Test Plan:
 * Import the flat manifest fixture file
 * There should be:
   * 2 discussions with titles that say you should see them
   * 1 assignment
	 * a module with 5 module items

closes PLAT-560

Change-Id: I975546e59dcdd731b2e0149c52fb7dd33f5e70e8
Reviewed-on: https://gerrit.instructure.com/37545
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: James Williams  <jamesw@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>