Enhance k3s_agent role to support conditional snapshotter for PXE-booted systems.

**Added:**

- PXE Boot Check - Introduced tasks to check if the system is PXE-booted by
  analyzing `/proc/cmdline` in `roles/k3s_agent/tasks/main.yml`.
- Conditional Snapshotter in Template - Added logic in `k3s.service.j2` template
  to conditionally set `--snapshotter native` for PXE-booted systems.

**Changed:**

- `k3s.service.j2` Template Update - Modified the `ExecStart` line to include a
  conditional check for `is_pxe_booted` fact, dynamically setting the
  `--snapshotter` option for NFS-mounted systems.
- `main.yml` Task Modification - Added tasks to set `is_pxe_booted` fact based
  on the presence of `root=/dev/nfs` in the system's boot command line.

This update allows k3s agents on PXE-booted systems to use the native snapshotter
when running on NFS, addressing compatibility issues with OverlayFS.

roles/k3s_agent/tasks/main.yml

@@ -1,4 +1,13 @@
 ---
+- name: Check if system is PXE-booted
+  command: cat /proc/cmdline
+  register: boot_cmdline
+  changed_when: false
+
+- name: Set fact for PXE-booted system
+  set_fact:
+    is_pxe_booted: "{{ 'root=/dev/nfs' in boot_cmdline.stdout }}"
+  when: boot_cmdline is defined
 
 - name: Deploy K3s http_proxy conf
   include_tasks: http_proxy.yml

roles/k3s_agent/templates/k3s.service.j2

@@ -7,11 +7,14 @@ After=network-online.target
 Type=notify
 ExecStartPre=-/sbin/modprobe br_netfilter
 ExecStartPre=-/sbin/modprobe overlay
-ExecStart=/usr/local/bin/k3s agent --server https://{{ apiserver_endpoint | ansible.utils.ipwrap }}:6443 --token {{ hostvars[groups[group_name_master | default('master')][0]]['token'] | default(k3s_token) }} {{ extra_agent_args | default("") }}
+# Conditional snapshotter based on PXE boot status
+ExecStart=/usr/local/bin/k3s agent \
+  --server https://{{ apiserver_endpoint | ansible.utils.ipwrap }}:6443 \
+  {% if is_pxe_booted | default(false) %}--snapshotter native \
+  {% endif %}--token {{ hostvars[groups[group_name_master | default('master')][0]]['token'] | default(k3s_token) }} \
+  {{ extra_agent_args | default("") }}
 KillMode=process
 Delegate=yes
-# Having non-zero Limit*s causes performance problems due to accounting overhead
-# in the kernel. We recommend using cgroups to do container-local accounting.
 LimitNOFILE=1048576
 LimitNPROC=infinity
 LimitCORE=infinity