Make OpenAI conversation model configurable via khoj.yml

- Default to using `text-davinci-003' if conversation model not
  explicitly configured by user. Stop using the older `davinci' and
  `davinci-instruct' models

- Use `model' instead of `engine' as parameter.
  Usage of `engine' parameter in OpenAI API is deprecated

config/khoj_docker.yml

@@ -51,4 +51,5 @@ search-type:
 processor:
   #conversation:
   #  openai-api-key: null
+  #  model: "text-davinci-003"
   #  conversation-logfile: "/data/embeddings/conversation_logs.json"

config/khoj_sample.yml

@@ -52,4 +52,5 @@ search-type:
 processor:
   conversation:
     openai-api-key: # "YOUR_OPENAI_API_KEY"
+    model: "text-davinci-003"
     conversation-logfile: "~/.khoj/processor/conversation/conversation_logs.json"