Ensure unit file folder is present in Debian 8

Some systems come with the /usr/lib/systemd/system directory. Some
others don't.
Ensuring with this commit that we always have this directory present,
otherwise the placing of the Docker unit file may fail and, in
consequence, the rest of the play.

tasks/main.yml

@@ -19,6 +19,11 @@
   notify:
     - restart docker
 
+- name: ensure unit file folder is present
+  file:
+    path: /usr/lib/systemd/system
+    state: directory
+
 - name: ensure unit file is present & up to date
   copy:
     src: docker.service