In nova, there is no way to explictly knows whether the neutron
fails due to over quota or no more floating ips and fixed ips.
so nova need to check whether the return value is 409 in order to
raise an over quota exception. It's easier in neutronclient to
raise the exception then nova will handle it.
Related-Bug: #1210598
Change-Id: I8788993578ac872da9f676fe3e2fb8f98414289d