fix Setting.from_config with no env
fix Setting.from_config with no env

test plan:
 * create a sms.yml, looking like:
Alltel: message.alltel.com
 * in script/console
 * CommunicationChannel.sms_carriers should return just Alltel

Change-Id: I7af78bc56d1ff083090ec8ab93c574dbe7a6b3da
Reviewed-on: https://gerrit.instructure.com/21841
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Brian Palmer <3cb4e1df5ec4da2c7c4af7c52cec8cf340a55a10@instructure.com>
QA-Review: Jeremy Putnam <b030d6d6f199f844ec424882be473c2ab9adc93b@instructure.com>
Product-Review: Brian Palmer <3cb4e1df5ec4da2c7c4af7c52cec8cf340a55a10@instructure.com>

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