Use directories.new when getting S3 directory

Calling `Fog::Storage::AWS::Directories#get` requires the
ListAllMyBuckets permission, but we can avoid that extra query and
permission by initializing the directory with a specific bucket:
https://stackoverflow.com/a/12288581/1992201

changelogs/unreleased/s3-directories-get.yml

+---
+title: Allow 'rake gitlab:cleanup:remote_upload_files' to read bucket files without
+  having permissions to see all buckets.
+merge_request: 23981
+author:
+type: fixed

lib/gitlab/cleanup/remote_uploads.rb

@@ -67,7 +67,7 @@ module Gitlab
       end
 
       def remote_directory
-        connection.directories.get(configuration['remote_directory'])
+        connection.directories.new(key: configuration['remote_directory'])
       end
 
       def connection

spec/lib/gitlab/cleanup/remote_uploads_spec.rb

@@ -25,7 +25,7 @@ describe Gitlab::Cleanup::RemoteUploads do
 
       expect(::Fog::Storage).to receive(:new).and_return(connection)
 
-      expect(connection).to receive(:directories).and_return(double(get: directory))
+      expect(connection).to receive(:directories).and_return(double(new: directory))
       expect(directory).to receive(:files).and_return(remote_files)
     end