pylxd/models/certificate: re-add password arg for backward compat

Fixes canonical/charm-lxd#168 where charm-lxd is calling certificates.create():

```python
  config: Dict[str, Union[str, bytes, List[str], bool]] = {
      "name": name,
      "password": "",
      "cert_data": cert.encode(),
  }

  client.certificates.create(**config)
```

causing:

```
  File "./src/charm.py", line 1139, in _on_https_relation_changed
    if self.lxd_trust_add(cert=cert, name=cert_name, projects=projects):
  File "./src/charm.py", line 2294, in lxd_trust_add
    client.certificates.create(**config)
TypeError: create() got an unexpected keyword argument 'password'
```

Signed-off-by: Simon Deziel <[email protected]>

pylxd/models/certificate.py

@@ -53,12 +53,13 @@ def all(cls, client):
     def create(
         cls,
         client,
-        secret,
+        password,
         cert_data,
         cert_type="client",
         name="",
         projects=None,
         restricted=False,
+        secret="",
     ):
         """Create a new certificate."""
         cert = x509.load_pem_x509_certificate(cert_data, default_backend())
@@ -73,9 +74,9 @@ def create(
             "projects": projects,
         }
         if client.has_api_extension("explicit_trust_token"):
-            data["trust_token"] = secret
+            data["trust_token"] = secret or password
         else:
-            data["password"] = secret
+            data["password"] = password
         response = client.api.certificates.post(json=data)
         location = response.headers["Location"]
         fingerprint = location.split("/")[-1]