Anbox Stream gateway charm got stuck at `Waiting for certificates` status all the time

[Gary.Wang]

Sporadically, the stream gateway gets stuck in the 'Waiting for certificates' status
```Unit Workload Agent Machine Public address Ports Message
anbox-stream-gateway/0* blocked idle 0 10.165.203.100 Waiting for certificates
easyrsa/0* active idle 1 10.165.203.230 Certificate Authority connected.
nats/0* active idle 0 10.165.203.100 4222/tcp
```

after a fresh deployment following by the error
```
  File "/var/lib/juju/agents/unit-anbox-stream-gateway-0/charm/./src/charm.py", line 234, in _on_tls_config_ready
    self.on.config_changed.emit()
  File "/var/lib/juju/agents/unit-anbox-stream-gateway-0/charm/venv/ops/framework.py", line 347, in emit
    framework._emit(event)
  File "/var/lib/juju/agents/unit-anbox-stream-gateway-0/charm/venv/ops/framework.py", line 857, in _emit
    self._reemit(event_path)
  File "/var/lib/juju/agents/unit-anbox-stream-gateway-0/charm/venv/ops/framework.py", line 947, in _reemit
    custom_handler(event)
  File "/var/lib/juju/agents/unit-anbox-stream-gateway-0/charm/lib/charms/tempo_coordinator_k8s/v0/charm_tracing.py", line 1064, in wrapped_function
    return callable(*args, **kwargs) # type: ignore
  File "/var/lib/juju/agents/unit-anbox-stream-gateway-0/charm/./src/charm.py", line 612, in _on_config_changed
    self._configure(start_now=False)
  File "/var/lib/juju/agents/unit-anbox-stream-gateway-0/charm/lib/charms/tempo_coordinator_k8s/v0/charm_tracing.py", line 1064, in wrapped_function
    return callable(*args, **kwargs) # type: ignore
  File "/var/lib/juju/agents/unit-anbox-stream-gateway-0/charm/./src/charm.py", line 634, in _configure
    if not cfg.nats.is_set and not self.set_from_relation("nats", cfg):
  File "/var/lib/juju/agents/unit-anbox-stream-gateway-0/charm/lib/charms/tempo_coordinator_k8s/v0/charm_tracing.py", line 1064, in wrapped_function
    return callable(*args, **kwargs) # type: ignore
  File "/var/lib/juju/agents/unit-anbox-stream-gateway-0/charm/./src/charm.py", line 721, in set_from_relation
    cfg.nats.urls.add(unit_data["url"])
  File "/var/lib/juju/agents/unit-anbox-stream-gateway-0/charm/venv/ops/model.py", line 1927, in __getitem__
    return super().__getitem__(key)
  File "/var/lib/juju/agents/unit-anbox-stream-gateway-0/charm/venv/ops/model.py", line 887, in __getitem__
    return self._data[key]
KeyError: 'url'
```