storage-driver: promote overlay2, make Btrfs and ZFS opt-in

The daemon uses a priority list to automatically select the best-matching storage
driver for the backing filesystem that is used.

Historically, overlay2 was not supported on Btrfs and ZFS, and the daemon would
automatically pick the `btrfs` or `zfs` storage driver if that was the Backing
File System.

Commits 649e4c88899878c9cdf9036f6bc7d62e2b39c04b and e226aea280efdc18f887e853b5eefd763b602937
improved our detection to check if overlay2 was supported on the backing file-
system, allowing overlay2 to be used on top of Btrfs or ZFS,  but did not change
the priority list.

While both Btrfs and ZFS have advantages for certain use-cases, and provide
advanced features that are not available to overlay2, they also are known
to require more "handholding", and are generally considered to be mostly
useful for "advanced" users.

This patch changes the storage-driver priority list, to prefer overlay2 (if
supported by the backing filesystem), and effectively makes btrfs and zfs
opt-in storage drivers.

This change does not affect existing installations; the daemon will detect
the storage driver that was previously in use (based on the presence of
storage directories in `/var/lib/docker`).

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

daemon/graphdriver/driver_linux.go

@@ -50,7 +50,7 @@ const (
 
 var (
 	// List of drivers that should be used in an order
-	priority = "btrfs,zfs,overlay2,fuse-overlayfs,aufs,overlay,devicemapper,vfs"
+	priority = "overlay2,fuse-overlayfs,btrfs,zfs,aufs,overlay,devicemapper,vfs"
 
 	// FsNames maps filesystem id to name of the filesystem.
 	FsNames = map[FsMagic]string{