generate instfs JWTs with HS512 algorithm
generate instfs JWTs with HS512 algorithm

closes RECNVS-390
refs RECNVS-387

test plan:
* smoke test that instfs still works

Change-Id: Id0a75fa25d293244e26d30c757b80f6c97f907f8
Reviewed-on: https://gerrit.instructure.com/150615
Reviewed-by: Jacob Fugal <f862f167b85d41b225785c70d70808bc7337c1fe@instructure.com>
Tested-by: Jenkins
QA-Review: Collin Parrish <aa381382ccdcc876f5cf7879d921ed03200f6804@instructure.com>
Product-Review: Xander Moffatt <27f70fe194da423604e6e0a63a3de19814736fa9@instructure.com>

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