[quickstart][RFE] Add configuration for multi-nic network isolation

[John Trowbridge]

The default libvirt network setup for quickstart does not provide enough bridges to test multi-nic network isolation. We need to provide a reference configuration showing how to do this.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/760174

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/760174
Committed: https://git.openstack.org/cgit/openstack/tripleo-quickstart/commit/?id=a8ff5de91c6e9e315afe5ebc2de108069e0e8f42
Submitter: Zuul
Branch: master

commit a8ff5de91c6e9e315afe5ebc2de108069e0e8f42
Author: Alexey Stupnikov <email address hidden>
Date: Wed Oct 28 16:43:49 2020 +0100

    Add option to add NAT networks to overcloud nodes

    Jinja template for overcloud VMs is currently configured to skip
    adding network to VM's definition if forward_mode: 'nat' is set
    for this network. As a result, libvirt networks with enabled NAT
    forwarding mode could not be used by overcloud nodes for external
    connectivity.

    It is not a problem for single-nic scenarios: director node is
    used to connect overcloud nodes to external networks. But this
    limitation makes it hard to create infrastructure for overcloud
    deployments with multiple NICs and network isolation.

    This patch adds flexibility by allowing users to force adding
    network connections to overcloud VMs even if forward_mode: 'nat'
    is set for this network. This patch doesn't change current
    behavior if force_ovc parameter is not defined for any network.

    Change-Id: Ibd2e40878b8fd4b1e4db7f654c251b4968da8b85
    Partial-Bug: #1666916