fix: name '_credentialFile' is not defined

This fixes an issue when CLIENT_ID, CLIENT_SECRET and REFRESH_TOKEN
are passed, or available from the environment.

lnetatmo.py

@@ -254,8 +254,9 @@ def renew_token(self):
             cred = {"CLIENT_ID":self._clientId,
                     "CLIENT_SECRET":self._clientSecret,
                     "REFRESH_TOKEN":self.refreshToken }
-            with open(self._credentialFile, "w") as f:
-                f.write(json.dumps(cred, indent=True))
+            if hasattr(type(self), "_credentialFile"):
+                with open(self._credentialFile, "w") as f:
+                    f.write(json.dumps(cred, indent=True))
         self._accessToken = resp['access_token']
         self.expiration = int(resp['expire_in'] + time.time())