Error initializing block storage client with Openstack 2026.1

#### Overview of the Issue

With the 2026.1 (Gazpacho) release, cinder dropped the `volumev3` endpoint.
It needs to be fetched from the endpoint list instead of hardcoded.

The error is `Error initializing block storage client: No suitable endpoint could be found in the service catalog.`

#### Reproduction Steps

After the upgrade of a cloud to 2026.1, `packer build` will fail when it previously worked:

```
# PACKER_LOG=1 /usr/bin/packer build openstack.pkr.hcl
(...)
openstack.openhpc: output will be in this color.

2026/08/25 15:40:35 Running builder: openstack
2026/08/25 15:40:35 [INFO] (telemetry) Starting builder openstack.openhpc
==> openstack.openhpc: Loading flavor: general.v1.small
2026/08/25 15:40:35 packer-plugin-openstack_v1.1.4_x5.0_linux_amd64 plugin: 2026/08/25 15:40:35 [INFO] Loading flavor by ID: general.v1.small
2026/08/25 15:40:35 packer-plugin-openstack_v1.1.4_x5.0_linux_amd64 plugin: 2026/08/25 15:40:35 [ERROR] Failed to find flavor by ID: Resource not found
2026/08/25 15:40:35 packer-plugin-openstack_v1.1.4_x5.0_linux_amd64 plugin: 2026/08/25 15:40:35 [INFO] Loading flavor by name: general.v1.small
==> openstack.openhpc: Verified flavor. ID: d7d3ab48-3948-4140-a690-60626ed3cabe
==> openstack.openhpc: Using existing SSH private key
==> openstack.openhpc: Using existing SSH private key
==> openstack.openhpc: Error initializing block storage client: No suitable endpoint could be found in the service catalog.
==> openstack.openhpc: Step "StepCreateVolume" failed
==> openstack.openhpc: [c] Clean up and exit, [a] abort without cleanup, or [r] retry step (build may fail even if retry succeeds)?
```

### Plugin and Packer version

Packer v1.16.0
packer-plugin-openstack v1.1.4

### Simplified Packer Buildfile

```
packer {
required_plugins {
openstack = {
version = ">= 1.0.0"
source = "github.com/hashicorp/openstack"
}
}
}
source "openstack" "openhpc" {

flavor = "en1.small"
use_blockstorage_volume = true
volume_type = "ssd"
volume_size = 20
volume_name = "my-image-xxx"

# Input image:
source_image = "xxxx"
source_image_name = "rl-98"

# Output image:
image_disk_format = "raw"
}

build {

source "source.openstack.openhpc" {
image_name = "my-image-xxx"
}

}
```

### Log Fragments and crash.log files

Openstack endpoints on 2025.1
```
$ openstack endpoint list | grep cinder
| uuid1 | RegionOne | cinderv3 | volumev3 | True | public | https://my.cloud.local/v3/%(tenant_id)s |
| uuid2 | RegionOne | cinder | block-storage | True | public | https://my.cloud.local/v3 |
| uuid3 | RegionOne | cinderv3 | volumev3 | True | internal | https://my.cloud.private.ip:8776/v3/%(tenant_id)s |
| uuid4 | RegionOne | cinder | block-storage | True | internal | https://my.cloud.private.ip:8776/v3
```

Openstack endpoints on 2026.1
```
| uuid5 | RegionOne | cinder | block-storage | True | public | https://my.cloud.private.ip:8776/v3 |
| uuid6 | RegionOne | cinder | block-storage | True | internal | https://my.cloud.private.ip:8776/v3
```

==> volumev3 endpoints have been removed, must switch to block-storage

Start at the StepCreateVolume failure and the block storage client initialization, then trace how the service catalog endpoint is selected. Reproduce with the supplied Packer build against OpenStack 2026.1; done means block storage initializes using the available block-storage endpoint instead of requiring volumev3.

Written by the indexing model from the issue text.