BigSwitch: Fix tenant_id for shared net requests

The URI port requests are sent to on the backend
contains the tenant_id of the network. This corrects
a bug where, on port updates and deletes, the tenant_id
of the port rather than the network was being used,
which was incorrect when attached to a shared network.

Closes-Bug: #1278530
Change-Id: I09ffc767c669416555867e975d0b7057a5cfa0cb
Merge "BigSwitch: Fix tenant_id for shared net requests"