add OPENAI_BASE_URL and OPENAI_MODEL config for self host

[ydf]

All Submissions:

• Are tests passing? (From the root-level of the repository please run pip install tox && tox)
  test it in prod
• I have added or updated a test to cover the changes proposed in this Pull Request
  no need
• I have updated the documentation to cover the changes proposed in this Pull Request
  yes

[mbi]

Thank you!