Code review comment for ~paul-meyer/cloud-init:fix-ntfs-mount

After discussion on IRC, made it so that

a.) if we can mount it, then check for data... don't reformat it if it has files
b.) if we cannot mount it... then check a config variable, if that variable says not to delete, then don't.

If we are able to mount and see that there is data on the disk (in NTFS fs), we'll emit a warning to guide the user to set the config setting.