Add support for changing Docker's storage driver

defaults/main.yml

@@ -3,3 +3,4 @@ consul_ip: 127.0.0.1
 docker_clusterstore: no
 dockerpy: no
 vagrant: no
+docker_storagedriver: devicemapper

files/docker.service

@@ -8,7 +8,7 @@ Type=notify
 # the default is not to use systemd for cgroups because the delegate issues still
 # exists and systemd currently does not support the cgroup feature set required
 # for containers run by docker
-ExecStart=/usr/bin/docker daemon -H unix:///var/run/docker.sock
+ExecStart=/usr/bin/docker daemon -H unix:///var/run/docker.sock -s {{ docker_storagedriver }}
 ExecReload=/bin/kill -s HUP $MAINPID
 LimitNOFILE=1048576
 LimitNPROC=1048576