Make sure leases are maintained until release

Dnsmasq asks nova-dhcpbridge for existing leases on reboot, but
we don't return addresses that have been deallocated but not yet
released. This modifies nova to return leased but not allocated
addresses. It also delays removing the vif until we disassociate
the ip so we still have access to the mac address.

Resolves-bug: 1300355

Change-Id: Id78d66aad5005f052a1b49002de0ef13ef5a298d
Merge "Make sure leases are maintained until release"