This allows users to create simple "profiles" via separate `invokeai.yaml` files.
- Remove `InvokeAIAppConfig.set_root()`, it's extraneous
- Remove `InvokeAIAppConfig.merge_from_file()`, it's extraneous
- Add `--config` to the app arg parser, add `InvokeAIAppConfig._config_file`, and consume in the config singleton getter
- `InvokeAIAppConfig.init_file_path` -> `InvokeAIAppConfig.config_file_path`