Misleading error message on validate when identity_endpoint is not set.

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#### Overview of the Issue

The parameter that is to be set is named `identity_endpoint`, but the error message is `Missing input for argument [auth_url]`.
#### Reproduction Steps

packer validate .

### Plugin and Packer version

packer v1.8.2

### Simplified Packer Buildfile

```
source "openstack" "otc_packer_image" {
}

build {
sources = ["source.openstack.otc_packer_image"]
}
```

### Operating system and Environment details

ubuntu 22.04

### Log Fragments and crash.log files

Screen output:
```
Error: 5 error(s) occurred:

* Missing input for argument [auth_url]
* An image_name must be specified
* An ssh_username must be specified
Note: some builders used to default ssh_username to "root".
* Either a source_image, a source_image_name, an external_source_image_url or source_image_filter must be specified
* A flavor must be specified

on test.pkr.hcl line 8:
(source code not available)
```