registry options dont work with Filebeat6

[peyerl-plusline]

The addition for registry options with PR #285 apparently does not work on Filebeat6.

The syntax Filbeat6 expects is:

filebeat.registry_file: $path/$filename
filebeat.registry_file_permissions: $perm
filebeat.registry_flush: $flush

but the syntax created here is:

filebeat:
  registry:
    path: $path
    file_permissions: $perm 
    flush: $flush

I don't have a Filebeat7 to test if it works there.