Add support for LXD server and VM composer

[AmitSahastra]

LXD Initializer DaemonSet for Secure VM Host Provisioning

Overview

This PR introduces a Kubernetes DaemonSet approach for LXD initialization and VM host registration. This approach addresses a critical reviewer comment about direct LXD socket access from the CAPMaaS controller and improves security and scalability in distributed environments.

Key Changes

• Added lxd-initializer DaemonSet that runs on each node to initialize LXD and register the node as a VM host with MAAS
• Updated pkg/maas/lxd/host.go to use MAAS API exclusively for VM host operations
• Enhanced pkg/maas/vmhosts package with comprehensive MAAS API client for VM host management
• Removed all direct LXD socket access from the CAPMaaS controller
• Added documentation for DaemonSet integration

Architecture

The new architecture separates concerns:

1. LXD Initializer DaemonSet: Runs on each node to initialize LXD and register with MAAS
2. CAPMaaS Controller: Uses MAAS API to create and manage VMs on registered VM hosts

This approach complements the existing LXD provisioning work by providing a secure and scalable method for LXD initialization while maintaining the comprehensive VM provisioning capabilities.

Benefits

• Security: No direct LXD socket access from controller
• Scalability: Works correctly in distributed environments
• Separation of Concerns: Clear separation between node-side initialization and controller-side VM provisioning
• API-Based: Uses MAAS API for all operations, no CLI commands

Testing

• Tested LXD initialization on Ubuntu 22.04 with LXD 5.x
• Verified VM host registration with MAAS API
• Confirmed compatibility with existing VM provisioning workflow

Documentation

• Added lxd-initializer/README.md with setup and usage instructions
• Added lxd-initializer/integration.md with integration guide
• Added lxd-initializer/SUMMARY.md with architectural overview