[Brightbox] Add CloudIp#destination_id

Abstraction over the number of possible mapped targets to consistently
access the identifier.

lib/fog/brightbox/models/compute/cloud_ip.rb

@@ -56,6 +56,10 @@ module Fog
           service.destroy_cloud_ip(identity)
         end
 
+        def destination_id
+          server_id || load_balancer || server_group || interface_id
+        end
+
       end
 
     end

tests/brightbox/models/compute/cloud_ip_tests.rb

@@ -0,0 +1,32 @@
+Shindo.tests("Fog::Compute[:brightbox] | CloudIp model", ["brightbox"]) do
+  pending if Fog.mocking?
+
+  @test_service = Fog::Compute[:brightbox]
+
+  tests("success") do
+    @server = Brightbox::Compute::TestSupport.get_test_server
+    server_id = @server.id
+
+    @cip = @test_service.cloud_ips.allocate
+
+    tests("#destination_id") do
+      returns(true) do
+        @cip.respond_to?(:destination_id)
+      end
+
+      @cip.map(@server)
+      @cip.wait_for { mapped? }
+
+      returns(server_id) do
+        @cip.destination_id
+      end
+
+      @cip.unmap
+      @cip.wait_for { !mapped? }
+    end
+
+    @cip.destroy
+
+    @server.destroy
+  end
+end