Fixes#7467
The -mserver argument line in volume-statefulset.yaml was missing a
trailing backslash, which prevented extraArgs from being passed to
the weed volume process.
Also:
- Extracted master server list generation logic into shared helper
templates in _helpers.tpl for better maintainability
- Updated all occurrences of deprecated -mserver flag to -master
across docker-compose files, test files, and documentation
