Quantum API _get_floating_ip_by_address mismatch with Nova-Net

_get_floating_ip_by_address() has a unspecified behavior when
IP address is empty and only one floating IP is allocated in
the pool. It should return a 404 Not found like Nova-Net does
but instead returns the only IP in the pool.

Fix bug #1190242

Change-Id: Id4843edda8e66ccabab5e9d053c595a1cd247770
Merge "Quantum API _get_floating_ip_by_address mismatch with Nova-Net"