Add driver option to enable/disable copy-on-write

[risteon]

Parse the driver option "copyonwrite" to enable/disable copy-on-write. The default "false" is unchanged.

Usage:

docker volume create -d anybox/buttervolume:latest \
  -o "copyonwrite=True" myvolume

Or in docker-compose.yml:

volumes:
  myvolume:
    driver: anybox/buttervolume
    driver_opts:
      copyonwrite: True

This is basically the same feature as #39 , just implemented with a driver option.

[risteon]

Thank you for this nice volume plugin. I just found it today and it's great!

[ccomb]

Thanks