Use an environment variable for the Azure OpenAI deployment

[snakedye]

Use an environment variable for the GPT deployment/version we want to use.

Tasks:

• [ ] Create environment variable in the .env.

• [ ] Use the environment variable to instantiate the GPT class.

  Acceptance Criteria:

• All the tests should pass.
• The default deployment should be the faster (gpt-35-turbo).
• Update the documentation.

[snakedye]

Closed by https://github.com/ai-cfia/fertiscan-backend/pull/81