[Core] Improve config_list_from_json (#1026)

* Improve config_list_from_json
The `env_or_file` variabe can point to an
environment variable of file path.

* Update autogen/oai/openai_utils.py

Co-authored-by: Chi Wang <wang.chi@microsoft.com>

* Use "with" to open config file

* Remove unused.

* Remove accidental added file

---------

Co-authored-by: Eric Zhu <ekzhu@users.noreply.github.com>
Co-authored-by: Chi Wang <wang.chi@microsoft.com>