fix files_external storage id migration

fixes migration issue reported in https://github.com/owncloud/core/issues/11585#issuecomment-59258091
The storage ID is meant to be unique for the same set of files. If you use different key/secret pairs to access the same bucket you will still see the same files. Mounting the same bucket twice will only give you the same files twice ... so reusing the same filecache is perfectly fine and the right thing to do.