abstract ssh credentials
abstract ssh credentials

closes: GRADE-2037

test plan
 - specs pass
 - EDU Shared/canvas-image-wip new jenkins build passes

Change-Id: Ifbba4ed6b38629639c24ab6e751b4f91b783ba5f
Reviewed-on: https://gerrit.instructure.com/182630
Tested-by: Jenkins
Reviewed-by: Rex Fleischer <360d56e185853f069087eaa5288d1374fc56d750@instructure.com>
QA-Review: Derek Bender <d24d8a7ee520e34b1cec38800225d9f36b552df7@instructure.com>
Product-Review: Derek Bender <d24d8a7ee520e34b1cec38800225d9f36b552df7@instructure.com>

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