Add 'arcyd list-repos' for listing repositories

Add a new command for querying repositories, this will make it easier to
script on top of arcyd as well as taking a step towards being able to
easily determine the status of arcyd from the command-line.

Test Plan:
$ ./precommit.sh
$ ./testbed/arcyd/test_shell.sh
    $ cd ../arcyd
    $ arcyd list-repos
    $ arcyd list-repos --only-formatted-repo-urls