Fix shell driver DriverOpts cross-contamination in secret creation

When creating a secret with driver=shell via the API, the file driver's
default DriverOpts (including path) were applied because DriverOpts was
empty. The shell driver rejects path as an unknown option, making it
impossible to create shell-driver secrets via the REST API or
podman-remote.

Only apply default DriverOpts from config when the requested driver
matches the configured default driver.

Signed-off-by: Joe Doss <[email protected]>