Provide a delayed delete capability for IPs

There should be an option to configure IPs for delayed deletion. This maps to Nova's delayed deletion capability. The goal is the allow the recovery of an instance or a service by preserving IPs for some time after the delete call is received. This would be implemented by:
1. Set the "deallocated_on" date (at the time of deallocation)
2. Get the cron job to delete ips whose deallocated_on date is older than the number of days we want to hold on to such ips.