qcow2 image creation

Hi, I'm trying to create openstack qcow2 images using packer.
Everything goes well, the image was created and i was able to create instances using default opensztack options.

The problem is that the instanced launched using this image fail if created using --boot-from-volume in the command for the server creation `openstack server create`.

**Error:**
`error: build of instance aborted: volume did not finish being created even after we waited 181 seconds or 19 attempts. and its status is error`

This does not happen if the image is first created in raw format and then converted to qcow2 later using `qemu-img convert`.

```
{
"builders": [
{
"type": "openstack",
"username": "*****",
"password": "******",
"identity_endpoint": "******",
"region": "",
"tenant_id": "",
"domain_name":"",
"image_name": "centos7-packer-test",
"ssh_username": "centos",
"source_image": "",
"availability_zone": "nova",
"flavor": "4",
"ssh_ip_version": "4",
"use_blockstorage_volume": "true",
"volume_size": "10",
"volume_availability_zone": "nova",
"image_disk_format": "qcow2",
"networks": [
""
]
}
],
"provisioners": [
{
"type": "shell",
"inline": ["sudo yum install -y vim"]
}

]
}
```
I wanted to ask, I need to set some additional variables in the json to enable the image to be launched using `--boot-from-volume` ? or is a error in the image format conversion in packer?