fix files_external storage id migration

fixes migration issue reported in https://github.com/owncloud/core/issues/11585#issuecomment-59258091
They are uniqe because the bucket name is also used as a subdomain, eg to host static website content like owncloud-jfd.s3-website-eu-west-1.amazonaws.com. owncloud-jfd is one of my test buckets ...