Remove db query from deallocate_fixed_ip

A previous patch updated deallocate_for_instance() in nova-network to
take the full instance object over rpc.  This patch passes that instance
through nova-network to deallocate_fixed_ip(), which allows us to avoid
N instance db lookups (where N is the number of fixed IPs being
deallocated).

Related to blueprint nova-network-objects

Change-Id: Ib92e824f1bea2caae668945fabfae997632cd5f0
Merge "Remove db query from deallocate_fixed_ip"