install a kernel, otherwise qemu devices don't work

lava-clean.yml

@@ -7,5 +7,6 @@
       - lava-server
       - apache2
       - lava-tool
+      - linux-image-amd64
       state: absent
       purge: yes

lava-install.yml

@@ -8,6 +8,7 @@
       name:
       - lava-server
       - lava-tool
+      - linux-image-amd64
       state: present
       default_release: stretch-backports
       update_cache: yes