praefect: Add list-storages subcommand

It would be handy for Praefct admins to easily print out the virtual storages with its associated storages without having to go into the config. This change adds a new subcommand list-storages that either prints out all virtual storages with their associated storages, or takes a virtual storage as a flag and prints out storages for that virtual storage.

fixes: #3239 (closed)

Changelog: added