switch to stretch as default image

roles/lxc_guest/defaults/main.yaml

@@ -1,5 +1,5 @@
 ---
 auto_start: true
 container_state: started
-distro: jessie
+distro: stretch
 vm_size: 5G

roles/lxc_guest/tasks/main.yaml

@@ -11,7 +11,7 @@
 
 - name: Check debian release
   assert:
-    that: distro in [ 'wheezy', 'jessie', 'stretch', 'sid', ]
+    that: distro in [ 'stretch', 'sid', 'buster' ]
     msg: "release {{ distro }} not supported by debian template"
 
 - block: