Fixes bug 1022804
This is phase 2 of the bug fix. This changeset adds support for dnsmasq
--dhcp-script to notify Quantum of lease renewals. Communication between
dnsmasq and the Quantum DHCP agent occurs via UNIX domain socket since dnsmasq
may run in a network namespace. The DHCP agent is responsible for
relaying the updated lease expiration back the Quantum server.
Change-Id: If42b76bbb9ec7543e681e26b9add8eb1d7054eeb
3fbdbf203b