Always include tenant_id in port delete request

Quantum does not include the tenant_id in port objects for floating IP addresses.
However, the Big Switch backend requires the tenant_id in the port removal requests.
This looks up the correct tenant_id whenever it is missing in the port removal.

Change-Id: I7580ed2f545ad2d92b831b65dfbb34541bf465aa
Fixes: bug #1190020
(cherry picked from commit f392263ae8c89412ec8be6a13b3dd56abd129a8c)