feat: add device_instances for target file sharing

[francoriba]

Add device_instances for sharing target files

Labs with multiple devices of the same model (e.g., 3x Belkin RT3200) had to duplicate target files or hardcode lab-specific configs, creating maintenance overhead.

Solution

Use device_instances in labnet.yaml to map multiple physical devices to a single target file:

labs:
  labgrid-example:
    devices:
      - linksys_e8450
    device_instances:
      linksys_e8450:
        - device_1
        - device_2
        - device_3

Creates 3 places, all using targets/linksys_e8450.yaml. Device-specific config (serial, IP, power) stays in exporter.

Key change

Automatic resolution: Setting LG_PLACE=labgrid-example-device_1 automatically resolves to LG_ENV=targets/linksys_e8450.yaml without any manual config needed.

Changes

• tests/conftest.py: Auto-resolution logic
• ansible/files/coordinator/places.yaml.j2: Template support
• docs/sharing-target-files.md: Full documentation
• README.md: Usage notes

Backward Compatible

Existing configs without device_instances would continue to work unchanged.