prepare_host.yaml: use python3 packages

`python3` and `python3-lxc` are installed with apt instead of `python`
and `python-lxc`.

prepare_host.yaml

@@ -14,8 +14,8 @@
         cache_valid_time: 3600
       with_items:
         - lxc
-        - python
-        - python-lxc
+        - python3
+        - python3-lxc
 
     - name: install common lxc filesystem support
       apt: