Fix an oversight in IStorageBackend.

Signed-off-by: Daira Hopwood <daira@jacaranda.org>
Add backend support for listing container contents. refs #1759

Signed-off-by: Daira Hopwood <daira@jacaranda.org>
Add the 'tahoe admin ls-container' command and tests. fixes #1759

Signed-off-by: Daira Hopwood <daira@jacaranda.org>