The charm doesn't balance between mailman3-web unit for hyperkitty

One solution is to have the charm install some kind of balancer, HAProxy, on the mailman3-core unit and configure backends to all the mailman3-web units from the relation data. Then have the template use this.

One other possibility here is that we could simply point to the external hostname for the service (e.g. https://lists.canonical.com/mailman3/hyperkitty/). We currently have a bundle deploying apache2 as a frontend for that, with the following:

    # Don't allow access to HyperKitty API from outside
    <Location /mailman3/hyperkitty/api>
        Require all denied
    </Location>

We could update this to something like:

    # Only allow access to admin from the openstack tenant this service runs in
    <Location /mailman3/hyperkitty/api>
        Require ip 10.x.x
    </Location>

This would need testing to confirm it allows internal traffic and not external traffic. Note that the "hostname" config option for this charm isn't the hostname of the service itself, but of the mailman3-core unit, so if we did take this approach we'd likely need to add another config option to the charm for that.