Fixes an issue where the stat check we were using on storage diretories was engish language dependant

Fixes: https://gitlab.com/gitlab-org/omnibus-gitlab/issues/1571

The printf filetype from stat, returning 'directory' when in english, returns other things in when language is set differently. Added a -d expression to the test statement.

Technically stat has raw output that we could have masked the bit and parsed to determine if it is a directory, but this should suffice.