Lack of clear instructions on how to get Sunbeam working in an environment with bridge interfaces on the Generic network

At the moment, the [Bootstrap highly available Juju controller on top of a LXD cluster](https://canonical-openstack.readthedocs-hosted.com/en/latest/how-to/misc/bootstrap-highly-available-juju-controller-on-top-of-a-lxd-cluster/) how-to guide requires bridge interfaces to be configured on the Generic network.

However, the [Install Canonical OpenStack using the manual bare metal provider](https://canonical-openstack.readthedocs-hosted.com/en/latest/how-to/install/install-canonical-openstack-using-the-manual-bare-metal-provider/) how-to guide does not work out of the box in an environment with bridge interfaces on the Generic network.

According to the engineering team, it is possible to get it working by overwriting some setting through the [Deployment manifest](https://canonical-openstack.readthedocs-hosted.com/en/latest/explanation/deployment-manifest/), but this is documented nowhere at the moment.

I'm suggesting updating the [Install Canonical OpenStack using the manual bare metal provider](https://canonical-openstack.readthedocs-hosted.com/en/latest/how-to/install/install-canonical-openstack-using-the-manual-bare-metal-provider/) how-to guide accordingly to warn users about this fact and provide them with all necessary information that would help them install Canonical OpenStack successfully.

Start with the manual bare metal provider installation guide and compare its assumptions with the highly available Juju controller guide. Read the deployment manifest documentation to identify the settings needed for bridge interfaces on the Generic network, then update the installation guide with a warning and the complete configuration guidance.

Written by the indexing model from the issue text.