Fix install-docker role

systemctl is not available on Ubuntu 14.04, so using
"service docker restart" as a workaround.

Related-Bug: theopenlab/openlab#308

roles/install-docker/tasks/main.yml

@@ -27,8 +27,14 @@
       iptables -P FORWARD ACCEPT
       iptables -P OUTPUT ACCEPT
 
-      # Restart docker.
-      systemctl daemon-reload
-      systemctl restart docker
+      # NOTE: systemctl is not available in Ubuntu 14.04
+      if [[ "{{ global_env.OS_BRANCH }}" == "stable/mitaka" ]];then
+          # Restart docker.
+          service docker restart
+      else
+          # Restart docker.
+          systemctl daemon-reload
+          systemctl restart docker
+      fi
 
     executable: /bin/bash