Add template parameter for storage credentials

The `python.jinja2` base template now has an optional `STORAGE_CONFIG_YAML` variable to enable jobs to upload files to storage. Each storage implementation has a different type of credentials, and this is initially hard-coded to handle private SSH keys via Kubernetes secret mounted as a file in `/home/kernelci/.ssh`. However, the name of the SSH key and the way the `STORAGE_CREDENTIALS` environment variable are being defined should depend on the storage configuration. So this will need more flexibility, using potentially a combination of additional YAML config attributes and Jinja2 templates based on the storage type.

Start with the python.jinja2 base template and trace how STORAGE_CONFIG_YAML, Kubernetes-mounted SSH keys, and STORAGE_CREDENTIALS are currently handled. Review the storage configuration and storage types mentioned by the issue before deciding how YAML attributes and Jinja2 templates should vary; done means credential names and environment-variable definitions work for each supported storage configuration.

Written by the indexing model from the issue text.