Port forwarding only from localhost interface

The current code configures port forwarding to the VM from all host
interfaces.

Most users probably don't want to expose their unsecured VMs to their
local network (or worse).

This patch limits port forwarding to the host's 127.0.0.1 interface.

Partial-Bug: 1312764
Implements: blueprint openstack-training-labs
Change-Id: Ib28439c728691510ac710544564e92193e66bf52