Builder Openstack image_min_disk

_This issue was originally opened by @possani as hashicorp/packer#9549. 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._

---

#### Overview of the Issue

Hi, I am trying to create a new image, but I am getting the following behavior:

When specifying the _image_min_disk_, I would expect to see what the [description](https://www.packer.io/docs/builders/openstack#image_min_disk) says, "Minimum disk size needed to boot image, in gigabytes"; instead, I am setting the disk size.


_image_min_disk_ and _image_min_ram_ are not available. Instead, the current option should be named _image_size_.

It also takes a very long time to save the image, maybe due to the image size.

#### Reproduction Steps

packer build template.json

### Packer version

Packer v1.6.0

### Simplified Packer Buildfile

```
{
"builders": [
{
"type": "openstack",
"ssh_username": "ubuntu",
"image_name": "Ubuntu-18.04-LTS-bionic-packer",
"image_min_disk": 20,
"source_image_name": "Ubuntu-18.04-LTS-bionic",
"insecure": "true",
...
"use_blockstorage_volume": "true"
}
]
}
```

### Operating system and Environment details

NAME="Ubuntu"
VERSION="20.04 LTS (Focal Fossa)"