Correct storage rebalance script rubocop linting

In support of #26 (closed), get rubocop passing for scripts/storage_rebalance.rb.

This commit will also support the specification of projects to migrate by one or more given project id identifiers, and also by a csv file list. So long as the project.repository_storage is the same as the specified source --current-file-server then only projects given by the command line argument --project=PROJECT_ID,... will be replicated to the given destination, --target-file-server.