populate dnsmasq lease db with valid leases

At start dnsmasq retrieves its lease db by calling the nova-dhcpbridge
script with the init action.

Previously we were returning all allocated IPs in the network and
calculating the lease exirpation time based upon instances.updated_at
or instances.created_at.

Now we are only returning entries for IPs that are allocated and leased.
Additionally we set the lease expiration time to be i
now + CONF.dhcp_lease_time

Change-Id: Ie3b4fafd38094dc01ceea592ab5229e581193512
Fixes: bug #1103260
(cherry picked from commit e12339def3d9182f5ab23869e6bdc946e8e42825)
Merge "populate dnsmasq lease db with valid leases" into stable/folsom