Attach additionnal block volumes to VM in Openstack

_This issue was originally opened by @gvcgael as hashicorp/packer#8098. It was migrated here as a result of the [Packer plugin split](https://github.com/hashicorp/packer/issues/8610#issuecomment-770034737). The original body of the issue is below._

Hello,

Currently, the openstack builder does not support adding additionnal blockstorage volumes to VM.

This is a case useful to add persistent data disks to VMs.

It is available on Azure builder ( https://www.packer.io/docs/builders/azure.html#disk_additional_size ) and AWS builder ( https://www.packer.io/docs/builders/amazon-instance.html#ami_block_device_mappings ) but not in Openstack builder.

There references API are :
- Create a volume : https://docs.openstack.org/api-ref/block-storage/v3/?expanded=upload-volume-to-image-detail#create-a-volume
- Attach a volume : https://docs.openstack.org/api-ref/block-storage/v3/?expanded=upload-volume-to-image-detail#attach-volume-to-a-server