PLUMgrid plugin: Fix for delete subnet with admin context

When delete call using admin for a subnet created from a
non-admin project is made, the tenant_id passed to backend
happened to be of admin project. This commit fixes the issues
by getting the correct tenant_id.

Closes-Bug: 1404688
Change-Id: Id21c38610ed73defb937d971a7aade57713541c0
Merge "PLUMgrid plugin: Fix for delete subnet with admin context"