Support defining a custom reboot command (#467)

Co-authored-by: Techno Tim <timothystewart6@gmail.com>

roles/lxc/handlers/main.yml

@@ -2,4 +2,5 @@
 - name: Reboot server
   become: true
   reboot:
+    reboot_command: "{{ custom_reboot_command | default(omit) }}"
   listen: reboot server

roles/raspberrypi/handlers/main.yml

@@ -1,4 +1,5 @@
 ---
 - name: Reboot
   reboot:
+    reboot_command: "{{ custom_reboot_command | default(omit) }}"
   listen: reboot