Pass '--dhcp-authoritative' option to dnsmasq

When dnsmasq is restarted, it forgets about all leases (since it runs
with leasefile-ro option). When client tries to renew its lease, dnsmasq
sends DHCPNAK reply with message "lease not found". Then client shuts
down the network and re-request lease from DHCP server (and gets exactly
same IP address). There's a small network downtime which affects
services, like zookeeper, running in VMs.

Change-Id: Ieff0236670c1403b5d79ad8e50d7574c1b694e34
Closes-Bug: #1345947
(cherry picked from commit 74a16fde1c)

neutron/agent/linux/dhcp.py

@@ -338,6 +338,7 @@ class Dnsmasq(DhcpLocalProcess):
             '--addn-hosts=%s' % self._output_addn_hosts_file(),
             '--dhcp-optsfile=%s' % self._output_opts_file(),
             '--leasefile-ro',
+            '--dhcp-authoritative',
         ]
 
         possible_leases = 0

neutron/tests/unit/test_linux_dhcp.py

@@ -697,7 +697,8 @@ class TestDnsmasq(TestBase):
             '--dhcp-hostsfile=/dhcp/%s/host' % network.id,
             '--addn-hosts=/dhcp/%s/addn_hosts' % network.id,
             '--dhcp-optsfile=/dhcp/%s/opts' % network.id,
-            '--leasefile-ro']
+            '--leasefile-ro',
+            '--dhcp-authoritative']
 
         expected.extend(
             '--dhcp-range=set:tag%d,%s,static,86400s' %