always search the account's shard for user list
always search the account's shard for user list

refs CNVS-18296

in case global lookups is enabled, but not populated; this fixes
that problem while not added any extra load in 99.99% of happy
cases

test plan:
 * configure global_lookups, but don't populate them
 * adding an existing user to a course should still
   work

Change-Id: If3012ee5e8b4bd2d79bee840fc1250fc43b7f186
Reviewed-on: https://gerrit.instructure.com/48796
Reviewed-by: Rob Orton <7e09c9d3e96378bf549fc283fd6e1e5b7014cc33@instructure.com>
Tested-by: Jenkins
Product-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
QA-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>

djbender/canvas-lms,grahamb/canvas-lms,whalejasmine/canvas-lms,eCNAP/CANVAS,whalejasmine/canvas-lms,redconfetti/canvas-lms,redconfetti/canvas-lms,snehakachroo1/LSI_replica,grahamb/canvas-lms,instructure/canvas-lms,HotChalk/canvas-lms,venturehive/canvas-lms,fronteerio/canvas-lms,antoniuslin/canvas-lms,antoniuslin/canvas-lms,skmezanul/canvas-lms,kyroskoh/canvas-lms,shidao-fm/canvas-lms,sdeleon28/canvas-lms,grahamb/canvas-lms,Jyaasa/canvas-lms,antoniuslin/canvas-lms,sfu/canvas-lms,va7map/canvas-lms,va7map/canvas-lms,wrdsb/canvas-lms,skmezanul/canvas-lms,dgynn/canvas-lms,kyroskoh/canvas-lms,ShreniiNepal/shrenii_lmp,Rvor/canvas-lms,sfu/canvas-lms,shidao-fm/canvas-lms,instructure/canvas-lms,venturehive/canvas-lms,jay3126/canvas-lms,leftplusrightllc/canvas-lms,HotChalk/canvas-lms,narigone/canvas,HotChalk/canvas-lms,narigone/canvas,sfu/canvas-lms,skmezanul/canvas-lms,leftplusrightllc/canvas-lms,roxolan/canvas-lms,roxolan/canvas-lms,SwinburneOnline/canvas-lms,utkarshx/canvas-lms,sdeleon28/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,eCNAP/CANVAS,SwinburneOnline/canvas-lms,jay3126/canvas-lms,venturehive/canvas-lms,snehakachroo1/LSI_replica,AndranikMarkosyan/lms,redconfetti/canvas-lms,samuels410/greatlakes-lms,Rvor/canvas-lms,samuels410/greatlakes-lms,efrj/canvas-lms,greyhwndz/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,tgroshon/canvas-lms,dgynn/canvas-lms,Jyaasa/canvas-lms,kyroskoh/canvas-lms,leftplusrightllc/canvas-lms,instructure/canvas-lms,va7map/canvas-lms,djbender/canvas-lms,matematikk-mooc/canvas-lms,efrj/canvas-lms,AndranikMarkosyan/lms,sfu/canvas-lms,grahamb/canvas-lms,tgroshon/canvas-lms,instructure/canvas-lms,wrdsb/canvas-lms,juoni/canvas-lms,utkarshx/canvas-lms,efrj/canvas-lms,wrdsb/canvas-lms,leftplusrightllc/canvas-lms,tgroshon/canvas-lms,shidao-fm/canvas-lms,jay3126/canvas-lms,ShreniiNepal/shrenii_lmp,matematikk-mooc/canvas-lms,sfu/canvas-lms,roxolan/canvas-lms,samuels410/greatlakes-lms,skmezanul/canvas-lms,redconfetti/canvas-lms,HotChalk/canvas-lms,juoni/canvas-lms,va7map/canvas-lms,roxolan/canvas-lms,greyhwndz/canvas-lms,greyhwndz/canvas-lms,djbender/canvas-lms,grahamb/canvas-lms,utkarshx/canvas-lms,AndranikMarkosyan/lms,juoni/canvas-lms,narigone/canvas,matematikk-mooc/canvas-lms,utkarshx/canvas-lms,efrj/canvas-lms,venturehive/canvas-lms,fronteerio/canvas-lms,SwinburneOnline/canvas-lms,kyroskoh/canvas-lms,eCNAP/CANVAS,greyhwndz/canvas-lms,shidao-fm/canvas-lms,matematikk-mooc/canvas-lms,ShreniiNepal/shrenii_lmp,narigone/canvas,juoni/canvas-lms,Jyaasa/canvas-lms,SwinburneOnline/canvas-lms,whalejasmine/canvas-lms,sfu/canvas-lms,Jyaasa/canvas-lms,samuels410/greatlakes-lms,sdeleon28/canvas-lms,Rvor/canvas-lms,AndranikMarkosyan/lms,ShreniiNepal/shrenii_lmp,dgynn/canvas-lms,whalejasmine/canvas-lms,instructure/canvas-lms,sdeleon28/canvas-lms,snehakachroo1/LSI_replica,tgroshon/canvas-lms,snehakachroo1/LSI_replica,eCNAP/CANVAS,wrdsb/canvas-lms,dgynn/canvas-lms,fronteerio/canvas-lms,fronteerio/canvas-lms,jay3126/canvas-lms,Rvor/canvas-lms,antoniuslin/canvas-lms