Need to pass port['port'] to _get_tenant_id_for_create()

Need to pass port['port'] to _get_tenant_id_for_create()

This change set I9ac44a8e moved this logic out of the transaction
which caused this bug. Unit tests did not catch this because
this error is caused on NVP since the tenant_id is passed in as a tag
and this change resulted in the tenant_id becoming None which is not
an allowed tag value. Will improve error handling and edge cases like
this to fake_nvpapiclient in different commit.

Change-Id: Ib0eaab1f389de98b335f58e9215981adbbf9e2ef