stable branches pull corresponding plugin stable branch
stable branches pull corresponding plugin stable branch

when using stable branches, pull all plugins from same branch.

Test Plan:
- Jenkins build passes
- Jenkins build from stable branch pulls stable branch plugins
- Jenkins plugin build from branch pull canvas and other plugins
  from same branch.
  - test with canvas-lms-refspec commit flag and without

Change-Id: I07d62f9c75413fe2a0c035b43a14634baa814eef
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/252250
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
QA-Review: James Butters <4c5a489c9bb72d41f20b3b22aaf4c9b864e32f20@instructure.com>
Product-Review: James Butters <4c5a489c9bb72d41f20b3b22aaf4c9b864e32f20@instructure.com>
Reviewed-by: Andrea Cirulli <b4e55ef5ee4bae8ad31f7c81009bfc0af1714a1f@instructure.com>

instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms