Implements set_hostname on ProxmoxSystemMixin:
1. Writes /etc/hostname (short name, base64-safe transfer)
2. Replaces old hostname in /etc/hosts via Python regex + base64
3. Updates /etc/mailname if present
4. Updates Postfix myhostname via postconf -e if installed
5. Applies hostname immediately at runtime via hostname(1)
6. Regenerates Proxmox node TLS certificate via pvecm updatecerts -f
(falls back to pvenode cert create → pveproxy restart)
Accepts bare hostname or FQDN. A reboot is required for the Proxmox
node name to update in the web UI / cluster — the driver logs this.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
