Use the list endpoint instead of ps to get ollama's models

[geronimo]

What does this PR do?

Change the endpoint used to determine what models are available in ollama from /api/ps to /api/tags

• [x] Addresses issue #332

Feature/Issue validation/testing/test plan

To reproduce the problem observed in the issue above:

• Start the ollama server (ensure there are compatible models installed)
• Ensure there is no current process running any model with ollama ps
• Start the llama-stack server and no model will be found, although the models are usable in ollama

After the change, there is no need to have a process running the models in Ollama. It'll pick them up anyway.

Before submitting

• [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• [x] Ran pre-commit to handle lint / formatting issues.
• [x] Read the contributor guideline, Pull Request section?
• [x] Updated relevant documentation.
• [ ] Wrote necessary unit or integration tests.