Use OpenAI standard environment variables

[gojira]

See here for reference on EVAR names, as well as pattern for using shell environment variables or inline Python os.environ settings. https://github.com/hwchase17/langchain/blob/master/docs/modules/llms/integrations/azure_openai_example.ipynb

Specifically:

OPENAI_API_KEY
OPENAI_API_TYPE
OPENAI_API_VERSION
OPENAI_API_BASE