magic-folder list should use the HTTP API instead of reaching into internal Magic-Folder state

[exarkun]

#205 introduces an HTTP API which will give back a list of all known Magic Folders and their filesystem locations. This should be all that magic-folder list needs to do its job. It should use this HTTP API instead of what it does now.

[exarkun]

I guess this is somewhat blocked on #200 because magic-folder list will need a way to find the HTTP API. I could add an explicit command-line argument but we probably just want it to find the magic-folder configuration and look there, right?

If there's anything accepted on the command line, it would be an alternate location for magic-folder configuration to load, I guess, not options to override each individual piece of configuration.