Fix cleanup on relation broken

[PietroPasotti]

Might fix canonical/traefik-k8s-operator#340

We need to discuss this a bit though, might want to test some more.

Manual test instructions:

juju deploy traefik-k8s --channel edge traefik
juju deploy self-signed-certificates --channel edge ssc
juju relate traefik:certificates ssc

# OK: 
curl https://<traefik APP IP> --insecure -v  # 404

juju remove-application ssc

juju deploy self-signed-certificates --channel edge ssc2
juju relate traefik:certificates ssc2

# ERROR
curl https://<traefik APP IP> --insecure -v  # ERROR error:0A000458:SSL routines::tlsv1 unrecognized name

# replace the lib

juju scp lib/charms/observability_libs/v1/cert_handler.py traefik/0:/var/lib/juju/agents/unit-traefik-0/charm/lib/charms/observability_libs/v1/cert_handler.py

juju remove-application ssc2

juju deploy self-signed-certificates --channel edge ssc3
juju relate traefik:certificates ssc3

# OK: 
curl https://<traefik APP IP> --insecure -v  # 404

[Abuelodelanada]

In the second check I'm getting:

$ curl https://192.168.1.250 --insecure -v                                                                                             35 ↵
*   Trying 192.168.1.250:443...
* Connected to 192.168.1.250 (192.168.1.250) port 443 (#0)
* ALPN, offering h2
* ALPN, offering http/1.1
* TLSv1.0 (OUT), TLS header, Certificate Status (22):
* TLSv1.3 (OUT), TLS handshake, Client hello (1):
* TLSv1.2 (IN), TLS header, Unknown (21):
* TLSv1.3 (IN), TLS alert, unrecognized name (624):
* error:0A000458:SSL routines::tlsv1 unrecognized name
* Closing connection 0
curl: (35) error:0A000458:SSL routines::tlsv1 unrecognized name

.