Improve automatic API keys naming in kernelci-core templates

https://github.com/kernelci/kernelci-core/pull/2634 introduces hack to allow defining keys over ENV variable.
This can be made better.
```
instance = os.getenv('KCI_INSTANCE', 'prod')
if instance == 'prod':
params['k8s_api_key'] = 'kci-api-jwt-early-access'
else:
params['k8s_api_key'] = 'kci-api-jwt-staging'
```

Start by reading pull request 2634 and the template code containing the shown KCI_INSTANCE-based API-key selection. Clarify the intended automatic naming behavior for production and staging, then update the templates and verify that each environment receives the expected key name.

Written by the indexing model from the issue text.