feat(ci): multi-node PVE provisioning and Docker-based shared storage

Provision two PVE nodes per version (a/b) for future cluster testing,
plus Docker-based iSCSI target and NFS server for shared storage tests.

Multi-node changes:
- Each PVE version gets two nodes: 9a/9b and 8a/8b (4 VMs total)
- Parameterized answer.toml FQDN for unique hostnames per node
- Per-node auto-install ISOs with unique answer files
- Node name discovered from FQDN and included in test config
- API token creation handles pre-existing tokens (delete + recreate)
- New test env vars: PVETEST_HOST_B, PVETEST_APITOKEN_B
- Removed preflight cleanup from provision (use explicit cleanup instead)

Docker storage services:
- New docker-compose.storage.yml with iSCSI (tgt) and NFS containers
- Host networking so PVE nodes can reach storage services
- Docker socket mounted into dev-infra container for host Docker access
- Docker CLI added to dev-infra Dockerfile stage
- New test env vars: PVETEST_STORAGE_VM_IP, PVETEST_ISCSI_IQN, PVETEST_NFS_EXPORT

Other fixes:
- first-boot.sh installs open-iscsi on PVE nodes
- preflight-cleanup.sh handles empty ISO filename gracefully
- TMPDIR set to work dir to avoid /tmp overflow during ISO uploads

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/Dockerfile.test

@@ -63,3 +63,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     && apt-get update && apt-get install -y --no-install-recommends \
        terraform proxmox-auto-install-assistant qemu-utils \
     && rm -rf /var/lib/apt/lists/*
+
+# Install Docker CLI (for managing storage containers on the host via mounted socket)
+RUN curl -fsSL https://get.docker.com | sh 2>/dev/null \
+    && rm -rf /var/lib/apt/lists/*