`frontend` service not changing `default_backend` on hook configuration

When applying the following code:

```
relation-set "services=[
{'server_options': ['maxconn 100', 'cookie S{i} check'], 'service_name': 'mindproject', 'servers': [['mindproject', '${host}', '${port}', '']], 'service_options': ['mode http', 'balance leastconn', 'cookie SRVNAME insert', 'option httpchk GET / HTTP/1.0']}]"
```

Default yaml service name remains after configuration change hook:

```
frontend mind-proxy
  default_backend haproxy_service
```

 Should be changed to after configuration change hook:

```
frontend mind-proxy
  default_backend mindproject
```