Use elevated context in resource_tracker.instance_claim

There's no requirement that the resource tracker be used by an admin
context, but the db updates for compute nodes won't work without an
admin context.

This updates instance_claim to match how resize_claim,
abort_instance_claim, drop_resize_claim, and update_usage function.

Needed by bp remove-cast-to-schedule-run-instance

Change-Id: Ib5b11ee6009f910966fbb849f020eb86c55de319
Merge "Use elevated context in resource_tracker.instance_claim"