dockerd: add docker-storage to init

dockerd: add docker-storage to init

Docker's backend storage driver can be configurable for certain
filesystems. The default is the overlay storage driver, but if you run
openwrt on a system with btrfs, this will allow you to override the
default configuration by settings the storage_driver in uci in dockerd's
global section. This value will be used in the created dockerd.json
file.

Signed-off-by: Keith T. Garner <redacted>