allow plugins to define additional periodic jobs
allow plugins to define additional periodic jobs

refs #CNVS-2333, #CNVS-2334, #CNVS-2335, #CNVS-2336

test plan:
n/a, this change doesn't do anything until a plugin defines some jobs

Change-Id: Ie7330f6f1d9e8704891dc729f7b304a55b506cdb
Reviewed-on: https://gerrit.instructure.com/16469
Reviewed-by: Brian Palmer <3cb4e1df5ec4da2c7c4af7c52cec8cf340a55a10@instructure.com>
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
QA-Review: Jon Jensen <44f878afe53efc66b76772bd845eb65944ed8232@instructure.com>

whalejasmine/canvas-lms,samuels410/greatlakes-lms,jay3126/canvas-lms,wrdsb/canvas-lms,SwinburneOnline/canvas-lms,arrivu/jigsaw-lms,greyhwndz/canvas-lms,SwinburneOnline/canvas-lms,Unow/ins,shidao-fm/canvas-lms,fronteerio/canvas-lms,matematikk-mooc/canvas-lms,wimemx/Canvas,antoniuslin/canvas-lms,dgynn/canvas-lms,usmschuck/canvas,redconfetti/canvas-lms,sfu/canvas-lms,sli7246/canvas_branch,ShreniiNepal/shrenii_lmp,matematikk-mooc/canvas-lms,jay3126/canvas-lms,landonwilkins/canvas-lms,whalejasmine/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,merichar/canvas-lms,wrdsb/canvas-lms,Unow/canvas-lms,djbender/canvas-lms,redconfetti/canvas-lms,shidao-fm/canvas-lms,kyroskoh/canvas-lms,utkarshx/canvas-lms,matematikk-mooc/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,efrj/canvas-lms,wimemx/Canvas,jay3126/canvas-lms,snehakachroo1/LSI_replica,va7map/canvas-lms,AndranikMarkosyan/lms,shidao-fm/canvas-lms,arrivu/arrivu-lms,snehakachroo1/LSI_replica,leftplusrightllc/canvas-lms,loganrun/pincan,leftplusrightllc/canvas-lms,merichar/canvas-lms,usmschuck/canvas,gbleydon/canvas-lms,nditech/red-innovadores-lms,arrivu/hoodemo,landonwilkins/canvas-lms,wrdsb/canvas-lms,Unow/canvas-lms,parbros/red-innovadores-lms,usmschuck/canvas,tgroshon/canvas-lms,samuels410/greatlakes-lms,sdeleon28/canvas-lms,efrj/canvas-lms,Rvor/canvas-lms,narigone/canvas,Unow/canvas-lms,snehakachroo1/LSI_replica,eCNAP/CANVAS,Rvor/canvas-lms,wye0220/shindig-canvas,instructure/canvas-lms,leftplusrightllc/canvas-lms,juoni/canvas-lms,djbender/canvas-lms,Unow/ins,arrivu/hoodemo,sfu/canvas-lms,narigone/canvas,rup/Tarrax,venturehive/canvas-lms,merichar/canvas-lms,jay3126/canvas-lms,rup/Tarrax,va7map/canvas-lms,va7map/canvas-lms,sfu/canvas-lms,gbleydon/canvas-lms,dgynn/canvas-lms,djbender/canvas-lms,redconfetti/canvas-lms,va7map/canvas-lms,landonwilkins/canvas-lms,greyhwndz/canvas-lms,grahamb/canvas-lms,venturehive/canvas-lms,parbros/red-innovadores-lms,HotChalk/canvas-lms,sfu/canvas-lms,efrj/canvas-lms,arrivu/jigsaw-lms,venturehive/canvas-lms,leftplusrightllc/canvas-lms,whalejasmine/canvas-lms,fronteerio/canvas-lms,arrivu/qibla-ecampus,Jyaasa/canvas-lms,sdeleon28/canvas-lms,roxolan/canvas-lms,grahamb/canvas-lms,eCNAP/CANVAS,ShreniiNepal/shrenii_lmp,skmezanul/canvas-lms,ShreniiNepal/shrenii_lmp,greyhwndz/canvas-lms,samuels410/greatlakes-lms,dgynn/canvas-lms,loganrun/pincan,redconfetti/canvas-lms,SwinburneOnline/canvas-lms,skmezanul/canvas-lms,roxolan/canvas-lms,Jyaasa/canvas-lms,snehakachroo1/LSI_replica,arrivu/jigsaw-lms,wimemx/Canvas,tgroshon/canvas-lms,HotChalk/canvas-lms,eCNAP/CANVAS,wrdsb/canvas-lms,nditech/red-innovadores-lms,kyroskoh/canvas-lms,ShreniiNepal/shrenii_lmp,sli7246/canvas_branch,kyroskoh/canvas-lms,sdeleon28/canvas-lms,instructure/canvas-lms,antoniuslin/canvas-lms,matematikk-mooc/canvas-lms,tgroshon/canvas-lms,arrivu/arrivu-lms,grahamb/canvas-lms,sfu/canvas-lms,utkarshx/canvas-lms,nditech/red-innovadores-lms,AndranikMarkosyan/lms,usmschuck/canvas,Unow/ins,whalejasmine/canvas-lms,grahamb/canvas-lms,sfu/canvas-lms,arrivu/qibla-ecampus,landonwilkins/canvas-lms,efrj/canvas-lms,narigone/canvas,tgroshon/canvas-lms,juoni/canvas-lms,Jyaasa/canvas-lms,Rvor/canvas-lms,SwinburneOnline/canvas-lms,wye0220/shindig-canvas,loganrun/pincan,utkarshx/canvas-lms,shidao-fm/canvas-lms,venturehive/canvas-lms,fronteerio/canvas-lms,instructure/canvas-lms,sli7246/canvas_branch,narigone/canvas,rup/Tarrax,HotChalk/canvas-lms,juoni/canvas-lms,dgynn/canvas-lms,instructure/canvas-lms,gbleydon/canvas-lms,juoni/canvas-lms,skmezanul/canvas-lms,grahamb/canvas-lms,antoniuslin/canvas-lms,parbros/red-innovadores-lms,AndranikMarkosyan/lms,kyroskoh/canvas-lms,greyhwndz/canvas-lms,arrivu/qibla-ecampus,Jyaasa/canvas-lms,instructure/canvas-lms,roxolan/canvas-lms,fronteerio/canvas-lms,skmezanul/canvas-lms,utkarshx/canvas-lms,HotChalk/canvas-lms,Rvor/canvas-lms,nditech/red-innovadores-lms,arrivu/arrivu-lms,arrivu/hoodemo,eCNAP/CANVAS,samuels410/greatlakes-lms,AndranikMarkosyan/lms,wye0220/shindig-canvas,roxolan/canvas-lms,antoniuslin/canvas-lms,sdeleon28/canvas-lms