[OpenStack] floating_ip is actually floating_ip_id

_This issue was originally opened by @gkoudjou as hashicorp/packer#8300. 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

According to the documentation, with the Openstack builders, we can defined a specific floating IP address.

https://www.packer.io/docs/builders/openstack.html#floating_ip

While testing, it appears that packer expects an ID.

#### Reproduction Steps

Here is some json file :

```
"builders": [
{
"type": "openstack",
"username": "{{user `username`}}",
"password": "{{user `password`}}",
"identity_endpoint": "XXXXXXXXX",
"region": "regionOne",
"image_name": "{{user `image_name`}}",
"instance_name": "packer-test",
"ssh_timeout": "5m",
"floating_ip_network": "external",
"floating_ip": "10.121.240.228",
]
}
],
```

And here is the result :
```
openstack output will be in this color.

==> openstack: Loading flavor: Tiny_Linux
openstack: Verified flavor. ID: 1
==> openstack: Not using temporary keypair
openstack: Found Image ID: d91327b6-ef25-4632-b866-8773927c6a83
==> openstack: Launching server...
==> openstack: Launching server...
openstack: Server ID: 805257ad-79fa-4e38-904c-ef1951e158bd
==> openstack: Waiting for server to become ready...
==> openstack: Error using provided floating IP '10.121.240.228': Resource not found
==> openstack: Terminating the source server: 805257ad-79fa-4e38-904c-ef1951e158bd ...
Build 'openstack' errored: Error using provided floating IP '10.121.240.228': Resource not found

==> Some builds didn't complete successfully and had errors:
--> openstack: Error using provided floating IP '10.121.240.228': Resource not found

```

But if a use the ID used by requesting openstack, it's working.

```
openstack output will be in this color.

==> openstack: Loading flavor: Tiny_Linux
openstack: Verified flavor. ID: 1
==> openstack: Not using temporary keypair
openstack: Found Image ID: d91327b6-ef25-4632-b866-8773927c6a83
==> openstack: Launching server...
==> openstack: Launching server...
openstack: Server ID: 324e2b3c-5f83-4829-ad6f-ce706f87f79a
==> openstack: Waiting for server to become ready...
openstack: Selected floating IP: '1d869a1f-9378-4069-a087-7050d337d8a3' (10.121.240.228)
==> openstack: Associating floating IP '1d869a1f-9378-4069-a087-7050d337d8a3' (10.121.240.228) with instance port...
openstack: Added floating IP '1d869a1f-9378-4069-a087-7050d337d8a3' (10.121.240.228) to instance!
```

### Packer version

1.5.0 (yesterday nightly build)

### Simplified Packer Buildfile

Please see above

### Operating system and Environment details

OS: RHEL 7.3
Architecture: x86_64

### Log Fragments and crash.log files

> openstack: Found Image ID: d91327b6-ef25-4632-b866-8773927c6a83
> ==> openstack: Launching server...
> ==> openstack: Launching server...
> openstack: Server ID: dc67299d-0523-44cf-8ac9-7495d6cd6b61
> 2019/10/31 14:45:31 packer_linux_amd64: 2019/10/31 14:45:31 server id: dc67299d-0523-44cf-8ac9-7495d6cd6b61
> 2019/10/31 14:45:31 packer_linux_amd64: 2019/10/31 14:45:31 Waiting for state to become: [ACTIVE]
> ==> openstack: Waiting for server to become ready...
> 2019/10/31 14:45:32 packer_linux_amd64: 2019/10/31 14:45:32 Waiting for state to become: [ACTIVE] currently BUILD (0%)
> 2019/10/31 14:45:34 packer_linux_amd64: 2019/10/31 14:45:34 Waiting for state to become: [ACTIVE] currently BUILD (0%)
> 2019/10/31 14:45:38 packer_linux_amd64: 2019/10/31 14:45:38 Waiting for state to become: [ACTIVE] currently BUILD (0%)
> 2019/10/31 14:45:41 packer_linux_amd64: 2019/10/31 14:45:41 Waiting for state to become: [ACTIVE] currently BUILD (0%)
> 2019/10/31 14:45:44 packer_linux_amd64: 2019/10/31 14:45:44 [INFO] Not using winrm communicator, skipping get password...
> ==> openstack: Error using provided floating IP '10.121.240.228': Resource not found
> ==> openstack: Terminating the source server: dc67299d-0523-44cf-8ac9-7495d6cd6b61 ...
> 2019/10/31 14:45:45 packer_linux_amd64: 2019/10/31 14:45:45 Waiting for state to become: [DELETED]
> 2019/10/31 14:45:46 packer_linux_amd64: 2019/10/31 14:45:46 Waiting for state to become: [DELETED] currently ACTIVE (0%)
> Build 'openstack' errored: Error using provided floating IP '10.121.240.228': Resource not found
>
> ==> Some builds didn't complete successfully and had errors:
> --> openstack: Error using provided floating IP '10.121.240.228': Resource not found
>
> ==> Builds finished but no artifacts were created.
> 2019/10/31 14:45:48 [INFO] (telemetry) ending openstack
> 2019/10/31 14:45:48 machine readable: error-count []string{"1"}
> ==> Some builds didn't complete successfully and had errors:
> 2019/10/31 14:45:48 machine readable: openstack,error []string{"Error using provided floating IP '10.121.240.228': Resource not found"}
> ==> Builds finished but no artifacts were created.
> 2019/10/31 14:45:48 [INFO] (telemetry) Finalizing.
> 2019/10/31 14:45:48 packer_linux_amd64: 2019/10/31 14:45:48 [INFO] 404 on ServerStateRefresh, returning DELETED
> 2019/10/31 14:45:48 [WARN] (telemetry) Error finalizing report. This is safe to ignore. Post https://checkpoint-api.hashicorp.com/v1/telemetry/packer: proxyconnect tcp: tls: first record does not look like a TLS handshake
> 2019/10/31 14:45:48 waiting for all plugin processes to complete...
> 2019/10/31 14:45:48 /home/vagrant/btp_base_image/packer_linux_amd64: plugin process exited
> 2019/10/31 14:45:48 /home/vagrant/btp_base_image/packer_linux_amd64: plugin process exited
>