Add --storage-opt graph driver option and pass through to driver

This lets you add storage specific options for the daemon.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)

daemon/graphdriver/btrfs/btrfs.go

@@ -22,7 +22,7 @@ func init() {
 	graphdriver.Register("btrfs", Init)
 }
 
-func Init(home string) (graphdriver.Driver, error) {
+func Init(home string, options []string) (graphdriver.Driver, error) {
 	rootdir := path.Dir(home)
 
 	var buf syscall.Statfs_t