feat(proxmox_lxc_provision): support custom bridge, interface name, and VLAN tag

The bridge (vmbr0) and interface name (eth0) were previously hardcoded
in the netif string, and there was no way to set an 802.1Q VLAN tag.
Expose lxc_bridge, lxc_iface_name, and lxc_vlan_tag (optional) so
containers can be attached to non-default bridges or tagged into a VLAN
without forking the role.

Also drop the misleading 'lxc_ipv6 | default(omit)' filter — default(omit)
does not produce omission inside a string context, and lxc_ipv6 always
has a default of 'auto' in defaults/main.yml.

roles/proxmox_lxc_provision/tasks/update.yml

@@ -8,7 +8,7 @@
     memory: "{{ lxc_memory }}"
     swap: "{{ lxc_swap }}"
     disk: "{{ lxc_disk }}"
-    netif: '{"net0": "name=eth0,gw={{ lxc_gateway }},ip={{ lxc_ipv4 }},ip6={{ lxc_ipv6 | default(omit) }},bridge=vmbr0"}'
+    netif: '{"net0": "name={{ lxc_iface_name }},bridge={{ lxc_bridge }},ip={{ lxc_ipv4 }},gw={{ lxc_gateway }},ip6={{ lxc_ipv6 }}{% if lxc_vlan_tag is defined %},tag={{ lxc_vlan_tag }}{% endif %}"}'
     pubkey: "{{ lookup('file', lxc_pubkey_file) | default(omit) }}"
     onboot: "{{ lxc_onboot | default(false) }}"
     startup: "{{ lxc_startup | default(omit) }}"