demos/terraform: Improve MicroCloud resource naming

[mseralessandri]

This PR avoids hardcoding the bridge variable name to microbr0, so deployments can use different network names such as microbr1 when needed.
It also supports custom VM numbering (for example micro5, micro6, micro7, micro8) with matching disk naming (i.e. local7, remote7...), making the demo reusable in environments where lower indexes are already occupied for multiple MicroCloud deployments on the same host.

Checklist

• I have read the contributing guidelines and attest that all commits in this PR are signed off, cryptographically signed, and follow this project's commit structure.
• I have checked and added or updated relevant documentation.

[Copilot]

Pull request overview

This PR updates the Terraform demo to make network and disk resource naming more configurable, aiming to support non-default bridge names and custom starting indices for VM/disk numbering so the demo can be reused across multiple deployments on the same host.

Changes:

• Added disk_number_start to support offset-based disk numbering.
• Refactored locals/resources to carry disk_number through to LXD volume names and storage-related outputs.
• Renamed the lxd_network Terraform resource label to avoid hardcoding microbr0.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
demos/terraform/variables.tf	Adds disk_number_start (recommended to validate as a positive integer).
demos/terraform/terraform.tfvars.example	Documents the new disk_number_start variable in example inputs.
demos/terraform/outputs.tf	Updates storage_layout output to include disk numbers in by-id paths (currently inconsistent with preseed inputs).
demos/terraform/main.tf	Adds per-system disk_number, updates LXD resource naming, and refactors Ceph disk selection logic.

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[roosterfish]

Thanks. I like that we don't anymore carry a vm_count as this is implied already by vm_names.