Add fixed_ip_disassociate attributes

Adding attributes for both fixed_ip_disassociate_timeout
and dhcp_lease_time. In nova-network on Folsom, setting
the disassociate timeout also requires modifying the lease
timeout as well, otherwise dnsmasq will disassociate the IP
when the lease expires.
Merge pull request #42 from DavidWittman/fixed-ip-disassociate

Add fixed_ip_disassociate_timeout attributes