Use information from the dnsmasq hosts file to call dhcp_release

Certain situations can cause the DHCP agent's local cache to get out
of sync with the leases held internally by dnsmasq.  This method of
detecting when to call dhcp_release is idempotent and not dependent on
the cache.  It is more robust.

Change-Id: I4eafd9cfb94a77a2f0229f89de5483dad23725cf
Closes-Bug: #1250644
(cherry picked from commit 965542bfac90194bd032e5e6aeb6a507dcb11088)