[openstack] add block_device_mapping_v2

fix block_device_mapping_v2

http://docs.openstack.org/api/openstack-compute/2/content/POST_block_device_mapping_v2_createServer__v2__tenant_id__servers_ext-os-block-device-mapping-v2-boot.html
:lipstick: The indentation here is rather un-Ruby-like, and (in my opinion) obscures the structure somewhat. Also, you're missing a hash inside the `:block_device_mapping_v2` array, and technically `:image_ref` is required, too. How about:

```ruby
vm = compute_client.servers.create(
  :name => name,
  :flavor_ref => flavor,
  :image_ref => image,
  :block_device_mapping_v2 => [
    {
      :device_name => "/dev/sda1",
      :source_type => "volume",
      :destination_type => "volume",
      :delete_on_termination => false,
      :uuid => cinder_uddi,
      :boot_index => 0
    }
  ]
)
```