Set tag depending on patchset or merge
Set tag depending on patchset or merge

for patchset, tag with refspec
for merge, tag with 'master'

test plan: build passes and tag is correct

Change-Id: I940f38f8cf13b45eb610e08381925d539664fd87
Reviewed-on: https://gerrit.instructure.com/182446
Tested-by: Jenkins
Reviewed-by: Derek Bender <d24d8a7ee520e34b1cec38800225d9f36b552df7@instructure.com>
QA-Review: Derek Bender <d24d8a7ee520e34b1cec38800225d9f36b552df7@instructure.com>
Product-Review: Derek Bender <d24d8a7ee520e34b1cec38800225d9f36b552df7@instructure.com>

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