Configurable default replication factor for virtual storages

Praefect currently supports configuring a replication factors on a per repository basis with the set-replication-factor sub-command. This require manual action from the admin for each repository. It's also not possible to configure a replication factor before a repository exists. This MR adds support for configuring a default replication factor per virtual storage. The default replication factor gets applied to every newly created repository in the virtual storage.

Default replication factor option is ignored if per_repository elector is not enabled as variable replication factor doesn't support any other election strategy. If a virtual storage has no default replication factor configured, Praefect doesn't store any assignments and keeps replicating each repository to every storage in a virtual storage.

Closes #3362 (closed)