Document instance_floating_ip_net attribute

This attribute selects a preferred network on which to associate the
instance floating ip.

website/source/docs/builders/openstack.html.md

@@ -153,6 +153,13 @@ builder.
 -   `insecure` (boolean) - Whether or not the connection to OpenStack can be
     done over an insecure connection. By default this is false.
 
+-   `instance_floating_ip_net` (string) - The ID of the network to which the
+    instance is attached and which should be used to associate with the floating
+    IP. This provides control over the floating ip association on multi-homed
+    instances. The association otherwise depends on a first-returned-interface
+    policy which could fail if the network to which it is connected is
+    unreachable from the floating IP network.
+
 -   `key` (string) - Client private key file path for SSL client
     authentication. If omitted the `OS_KEY` environment variable can be used.