Openstack builder connects to wrong endpoint

_This issue was originally opened by @JoelFeiner as hashicorp/packer#5201. 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._

I am attempting to build an image using the OpenStack provider. It ultimately fails at launching the server with the following message: "Error launching source server: Invalid request due to incorrect syntax or missing required parameters".

It is able to connect to the identity service, verify the flavor and set up the keypair (which might be a local thing).

One thing I am noticing is that tcpdump shows a lot of connections to ec2-54-235-77-197.compute-1.amazonaws.com, whereas when I launch the server with the nova client, there are none. I'm wondering if there's a hardcoded URL in Packer somewhere.

I'm unable to further debug this because Packer or the underlying library does not provide the response from the OpenStack server. From Googling, I see that this is simply not supported at the current juncture.

Packer 1.0.3
Ubuntu 17.04 64-bit
Build output: https://gist.github.com/JoelFeiner/7f7102e3324e4efc00980600217a07aa

Builder config in JSON file:
```
{
"type": "openstack",
"flavor": "A1.2",
"source_image": "7925b8a6-b439-4991-9adf-092a202ff16e",
"ssh_username": "root",
"networks": [ "bf7a7067-8739-4421-9a08-c940d7fd435d" ],
"image_name": "{{user `image_type`}}-{{user `image_verson`}}"
}
```

Environment variables set:
OS_AUTH_URL
OS_REGION_NAME
OS_IDENTITY_API_VERSION
OS_TENANT_NAME
OS_PASSWORD
OS_USERNAME