Deny API requests where context doesn't match path

We shouldn't overwrite the context tenant_id (which comes from the
scope of the auth_token) with that from the path, instead raise a
HTTPForbidden exception if the path-provided tenant_id doesn't match
the context.

Change-Id: Ib6fb9881103312f7492081a20178f12309f35d81
Closes-Bug: #1256983
Merge "Deny API requests where context doesn't match path" into stable/havana