fix: resolve Docker Compose compatibility and firewall SSH blocking issues

Main fixes:
- Fix firewall configuration blocking SSH during cloud-init (root cause)
- Install Docker Compose V2 plugin instead of standalone version
- Update integration tests to auto-detect compose command

Changes:
- infrastructure/cloud-init/user-data.yaml.tpl:
  * Remove docker-compose package, install Docker Compose V2 plugin
  * Improved firewall setup to allow SSH throughout process
- infrastructure/tests/test-integration.sh:
  * Add get_docker_compose_cmd() helper for compatibility
  * Auto-detect 'docker compose' vs 'docker-compose' commands
  * Update all compose operations to use detected command
- docs/guides/integration-testing-guide.md:
  * Add comprehensive integration testing guide
  * Correct timing estimates (8-12 min total, 2-3 min cloud-init)
  * Document firewall fix as main solution
  * Add Docker Compose compatibility notes

Result: VM deployment now completes reliably in 2-3 minutes with proper
SSH connectivity throughout the cloud-init process.

Author: josecelano