supress dhcp router opt for subnets with null gw

fixes bug 1045617

This patch supresses the default gateway for subnets without gateway
defined.

Change-Id: I29d84863e51f613d8529153870eb7542d992be0e

quantum/tests/unit/test_linux_dhcp.py

@@ -95,6 +95,16 @@ class FakeV4SubnetNoDHCP:
     dns_nameservers = []
 
 
+class FakeV4SubnetNoGateway:
+    id = 'eeeeeeee-eeee-eeee-eeee-eeeeeeeeeeee'
+    ip_version = 4
+    cidr = '192.168.1.0/24'
+    gateway_ip = None
+    enable_dhcp = True
+    host_routes = []
+    dns_nameservers = []
+
+
 class FakeV4Network:
     id = 'aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa'
     subnets = [FakeV4Subnet()]
@@ -119,6 +129,12 @@ class FakeDualNetworkSingleDHCP:
     ports = [FakePort1(), FakePort2(), FakePort3()]
 
 
+class FakeV4NoGatewayNetwork:
+    id = 'cccccccc-cccc-cccc-cccc-cccccccccccc'
+    subnets = [FakeV4SubnetNoGateway()]
+    ports = [FakePort1()]
+
+
 class TestDhcpBase(unittest.TestCase):
     def test_base_abc_error(self):
         self.assertRaises(TypeError, dhcp.DhcpBase, None)
@@ -460,6 +476,16 @@ tag:tag0,option:router,192.168.0.1""".lstrip()
 
         self.safe.assert_called_once_with('/foo/opts', expected)
 
+    def test_output_opts_file_no_gateway(self):
+        expected = "tag:tag0,option:router"
+
+        with mock.patch.object(dhcp.Dnsmasq, 'get_conf_file_name') as conf_fn:
+            conf_fn.return_value = '/foo/opts'
+            dm = dhcp.Dnsmasq(self.conf, FakeV4NoGatewayNetwork())
+            dm._output_opts_file()
+
+        self.safe.assert_called_once_with('/foo/opts', expected)
+
     def test_reload_allocations(self):
         exp_host_name = '/dhcp/cccccccc-cccc-cccc-cccc-cccccccccccc/host'
         exp_host_data = """