Rename `openai_api` to `openai_api_key`

[nonprofittechy]

In llms.py, we frequently refer to a configuration value of openai_api. This is a nonstandard parameter name and is hard to remember.

we need to make it fully backwards compatible, so we should recognize the old parameter. There is no official need to deprecate it yet.

We should handle a few variations:

• There is a global configuration option visible via get_config() that we should standardize, open ai: org and open ai: key that should work when specified as openai: org and openai: key, as the space is not actually part of the name.
• we also have the existing global configuration openai api key: without an org that should remain valid.

[nonprofittechy]

Also see openai api key: sk... which we might want to keep as a fallback--it's in some configurations but llms.py doesn't look for it.

This should be the default, highest priority global option that we look for in llms.py