Enable the internal IP as an allowed host for the cos-registration-server

[Guillaumebeuzeboc]

For the charm to request the public-ssh-keys as well as the available Grafana dashboards, an http request is made.
This http request could be made to the internal IP. This will make the request versatile when traefik is down.

Currently, we only provide and use the external IP.
This might require changes on the cos-registration-server side for it to accept multiple IPs.