Fix FIP status inconsistency

This patch fixes the failuer of test_network_basic_ops in
tempest. update_floatingip would set the appropriate status
based on the situation if the FIP is associated or disassociated
with a port.

This fixes MN-2995.

Change-Id: I84d6c92b623562559d300cd5afa967b30088a643
Signed-off-by: Taku Fukushima <tfukushima@midokura.com>