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