Support other storages than `default`
Currently, HGitaly servers only accept repositories with storage_name=default
.
Actual sharding in HGitaly is not on the table yet, and serving several storages with a single HGitaly server even less, but we might soon want to have several storages in a Heptapod installation, with a single one for Mercurial repositories.
In any case, even with a single storage per HGitaly server, it would have to match the name as known in the Rails application, hence potentially not 'default'.
It would be good enough for the foreseeable future to configure both the repositories root and the storage name. We don't need a lookup in a full mapping.