Reverse the priority of floating_ip & floating_ip_pool

In my case, I would like to have the ability to override the `floating_ip_pool` option with the `floating_ip` option.
I wanna have one suite running on an instance with the floating_ip I explicitely provided only for this suite. And the rest of the suites using the default setting which is retreiving a random ip from the floating_ip_pool.

my `.kitchen.yml` example
```
---
driver_config:
  require_chef_omnibus: true

driver:
  name: openstack
  openstack_username: "*********"
  openstack_api_key: "*********"
  openstack_auth_url: "http://mystack.com:5000/v2.0/tokens"
  openstack_identity_endpoint: "http://mystack.com:5000/v2.0"
  openstack_management_url: "http://mycontroller:8774/v2/****************"
  username: "myuser"
  private_key_path: "/mykey.pem"
  key_name: "my-key"
  require_chef_omnibus: true
  image_ref: ubuntu1204
  flavor_ref: m1.testkitchen
  security_groups:
    - default
  floating_ip_pool: ext-net
  network_ref:
    - test-kitchen-network

provisioner: chef_zero

platforms:
  - name: ubuntu
    run_list:

suites:
  - name: uv_apache
    run_list: 
      - recipe[uv_apache::default]
      - recipe[minitest-handler]
    attributes:
      nfs:
        nfs_server_ip: '10.11.11.63'
  - name: uv_nfs_server
    run_list:
      - recipe[uv_nfs::default]
    attributes:
      nfs:
        network: '10.11.11.0/24'
    driver:
      floating_ip: '10.11.11.63'
```

Thank you