Make `docker volume` behave like `docker network`

Before, typing `docker volume` with no args would forward to the handler
for `docker volume ls`, except the flags for the `ls` subcommand were
not supported.
Instead just print the cmd usage.

This makes the behavior of the `docker volume` subcommand behave exactly
like the `docker network` subcommand.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>