bug with floatingip clear

[klonhj2015]

Now we delete a port ,then the plugin will clear floatingip. But in the clear method, it just update field what we want.It's very good ,but it will accur a problem with gson. For that gson will not serialize null field, so if we clear a floating ip,we want to set FixedAdrress with null value , contrail api will ignore the FixedIpAddress field.