Fix MeteringLabel model to not clear router's tenant id on deletion

foreign_keys parameter of orm.relationship should point to local columns.
Currently for MeteringLabel it points to Router.tenant_id column which causes
routers tenant_id clearing on label deletion.

Closes-Bug: #1249188
(cherry picked from commit d1220a3c22fccc1d86eaafeebe7ef9c074d4fcb9)
Change-Id: Iccc0daf4f6edd537fd7f9e4b2fc4be094543ca5d
Merge "Fix MeteringLabel model to not clear router's tenant id on deletion" into stable/havana