Configure a default replication factor for a virtual storage

Currently, the only way to set a replication factor in Praefect is to use the set-replication-factor subcommand. This is per repository basis and can't be used prior to the repository existing. Praefect should support configuring a default replication factor for a virtual storage that gets applied to every newly created repository.

Release notes

Gitaly Cluster now supports configuring a replication factor on a per-repository basis, by allowing you to assign specific storage nodes to host a repository. This is crucial to the continued maturity of Gitaly Cluster as it allows users to horizontally scale their cluster through adding nodes, and configuring newly created repositories to utilize these new nodes.

https://docs.gitlab.com/ee/administration/gitaly/praefect.html#configure-replication-factors