Configuration format for shard->path map

For sharding gitaly will need to be able to map something like this:

Repository.new(shard: 'storage01', path: 'foo/bar.git')

To:

/mnt/st1/repositories/foo/bar.git

In other words, in this example we'd want a map storage01 -> '/mnt/st1/repsitories'.

How will we pass this configuration into the Gitaly server process? Our current (and only) configuration mechanism is environment variables.