fix background image path var for maintenance mode

templates/lightdm-gtk-greeter.conf.j2

@@ -47,7 +47,7 @@
 #  transition-duration = overrides default value
 #
 [greeter]
-background=/usr/share/lightdm/background/{{display_manager_background_image}}
+background={{display_manager_background_image_path}}
 user-background=false
 #theme-name=
 #icon-theme-name=

vars/main.yml

@@ -0,0 +1 @@
+display_manager_background_image_path: /usr/share/lightdm/background/{{display_manager_background_image}}