Add fixed_ip_disassociate_timeout attributes

Adding attributes for both `fixed_ip_disassociate_timeout` and `dhcp_lease_time`. In nova-network on Folsom, setting the disassociate timeout also requires the following changes:

* `node["nova"]["network"]["force_dhcp_release"] = false`, otherwise the IP will be released and disassociated immediately
* `node["nova"]["network"]["dhcp_lease_time"]` must be increased to at least the value of `fixed_ip_disassociate_timeout`, otherwise the IP address will be disassociated when dnsmasq expires the lease.