monitoring: Fix network name mismatch

[michaelstingl]

monitoring/monitoring.yml declares opencloud-net as external:

networks:
  opencloud-net:
    external: true

But docker-compose.yml creates the network with project prefix (e.g. opencloud_opencloud-net when cloned to /opt/opencloud/).

PR #30 documented a workaround (docker network create opencloud-net), but this requires manual setup before first start.

Fix

Add explicit name to docker-compose.yml:

networks:
  opencloud-net:
    name: opencloud-net

This prevents the project prefix and matches the external: true reference.