Comment 3 for bug 1843841

When using the dns-slaves (external-slaves) it may be required to specify which interface to advertise to the slaves. In John's case, a routable address is required.

We appear to be hardcoding the internal space for the RNDC master [0]
{% for rndc_master_ip in cluster.internal_addresses %}

Test and validate:
Pass the dns-backend space addresses to the context
Test without a dns-backend relation attached.

If that does not work, we may need to create an external binding for this purpose.

[0] https://github.com/openstack/charm-designate/blob/master/src/templates/rocky/pools.yaml#L47