OpenAI base model initialization ignores currently set openi.api_key
The current design leaves only two options for initialization `openai-python`:

1. `OPENAI_API_KEY` environment variable
2. pass in `openai_api_key`

LangChain works just fine with the other `openai-python` settings like `openai.api_type`.

Can we make it so it uses `openai.api_key` if it is not `None`?

Happy to try a PR but wanted to understand design choice first.

https://github.com/hwchase17/langchain/blob/master/langchain/llms/openai.py#LL202C9-L202C29 