fix(config): revised config methods

- `write_file` requires an destination file path
- `read_config` -> `merge_from_file`, if no path is provided, reads from `self.init_file_path`
- update app, tests to use new methods
- fix configurator, was overwriting config file data unexpectedly

invokeai/app/api_app.py

@@ -5,7 +5,7 @@ from invokeai.app.services.config.config_default import get_config
 
 app_config = get_config()
 app_config.parse_args()
-app_config.read_config()
+app_config.merge_from_file()
 
 if True:  # hack to make flake8 happy with imports coming after setting up the config
     import asyncio