fix(docker): use btrfs as coreos does.

When using aufs in docker on coreos, obscure errors show up, such
as python's gevent not building for autoconf errors. Specifying
btrfs as the filesystem fixes it.
See moby/moby#2056

Author: mboersma

contrib/coreos/user-data

@@ -15,7 +15,7 @@ coreos:
       ExecStartPre=/bin/mount --make-rprivate /
       # Run docker but don't have docker automatically restart
       # containers. This is a job for systemd and unit files.
-      ExecStart=/usr/bin/docker -d -s=aufs -r=false -H fd://
+      ExecStart=/usr/bin/docker -d -s=btrfs -r=false -H fd://
 
       [Install]
       WantedBy=multi-user.target