add NVIDIA_BASE_URL and NVIDIA_API_KEY to control hosted vs local endpoints (#897)

# What does this PR do?

allows template distribution connect to hosted or local NIM:

use --env NVIDIA_BASE_URL=http://localhost:8000 to connect to a local
NIM running at localhost:8000

use --env NVIDIA_API_KEY=blah when connecting to hosted NIM, e.g.
NVIDIA_BASE_URL=https://integrate.api.nvidia.com


## Test Plan

- `llama stack run ./llama_stack/templates/nvidia/run.yaml` -> error,
e.g. API key is required for hosted NVIDIA NIM
- `llama stack run ./llama_stack/templates/nvidia/run.yaml --env
NVIDIA_BASE_URL=https://integrate.api.nvidia.com` -> error, e.g. API key
is required for hosted NVIDIA NIM

- `llama stack run ./llama_stack/templates/nvidia/run.yaml --env
NVIDIA_API_KEY=REDACTED` -> successful connection to NIM on
https://integrate.api.nvidia.com
- `llama stack run ./llama_stack/templates/nvidia/run.yaml --env
NVIDIA_BASE_URL=https://integrate.api.nvidia.com --env
NVIDIA_API_KEY=REDACTED` -> successful connection to NIM running on
integrate.api.nvidia.com

- `llama stack run ./llama_stack/templates/nvidia/run.yaml --env
NVIDIA_BASE_URL=http://localhost:8000` -> successful connection to NIM
running on localhost:8000
- `llama stack run ./llama_stack/templates/nvidia/run.yaml --env
NVIDIA_BASE_URL=http://localhost:8000 --env NVIDIA_API_KEY=REDACTED` ->
successful connection to NIM running on http://localhost:8000

- `llama stack run ./llama_stack/templates/nvidia/run.yaml --env
NVIDIA_BASE_URL=http://bogus` -> runtime error, e.g. ConnectionError
(TODO: this should be a startup error)


## 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](https://github.com/meta-llama/llama-stack/blob/main/CONTRIBUTING.md),
      Pull Request section?
- [ ] Updated relevant documentation.
- [ ] Wrote necessary unit or integration tests.

llama_stack/templates/nvidia/run.yaml

@@ -15,8 +15,8 @@ providers:
   - provider_id: nvidia
     provider_type: remote::nvidia
     config:
-      url: https://integrate.api.nvidia.com
-      api_key: ${env.NVIDIA_API_KEY}
+      url: ${env.NVIDIA_BASE_URL:https://integrate.api.nvidia.com}
+      api_key: ${env.NVIDIA_API_KEY:}
   vector_io:
   - provider_id: faiss
     provider_type: inline::faiss