Fixes update router gateway successful with existed floatingip association

Fixes bug 1081877

Change-Id: I77e6034a39dc9db85de691a9ba596b2c8ca9d124
Merge "Fixes update router gateway successful with existed floatingip association" into stable/folsom
Merge remote-tracking branch 'openstack/stable/folsom' into folsom/master
Merge pull request #3 from nbartos/folsom/master

Merge from upstream openstack/stable/folsom