Commit quota when deallocate floating ip

It seems that when you allocate a floating-ip in a tenant with nova-network,
its quota is never returned after calling 'nova floating-ip-delete'
even though 'nova floating-ip-list' shows it gone.
This behavior applies to each tenant individually. The gate tests are passing
because they all run with tenant isolation.

The root cause of the problem is cooperation between
commit 23a27e47 and cbbb9de5. db layer code return floatingip_ref
but object layer didn't. This patch fixed the problem by adding return
value from object layer.

Change-Id: Ide1a338b6c33676311028e8738150e146324a8ee
Closes-Bug: #1347156
Merge "Commit quota when deallocate floating ip"