Expand Praefect Rake task to print out checksum of repositories and their replicas

Proposal

During customer emergency we had to use the praefect rake task to identify projects that had a mismatch on their checksums. This could not be done with other methods due to praefect DB being on a bad state.

The current rake task allows to check single repository. This feature request is to expand the task to allow to run for all repositories.

Use same rake task sudo gitlab-rake "gitlab:praefect:replicas[project_id]" but do not provide [project_id] to allow it to run for all projects.

Also document the usage of this task