parser.Stack accepts tenant_id as a param

Previously, parser.Stack was exclusively using the tenant_id from the
request context.  However, when we load an existing stack, it makes
sense to use the existing tenant information from the stack in the db
(the source of truth) rather than the tenant info from the request
context.

Change-Id: Ic12149274bc35eca193da0b3acdbde4df92f3be4