fix kury-cni exits unexpectedly when MTU of network is 0

If the MTU of the netowrk of interface is 0,
"Netlink Error, invalid argument" will be raised,
and kury-cni exits unexpectedly.

When mtu is 0,
1. dhcp-option-force of dnsmasq for is not set,
   and mtu of interface of vm is set to be 1500 by default
2. mtu of qvo-xxx and qvb-xxx is set to 1500,
   which is determined by configuration,network_device_mtu.

Fix this by adding default network device MTU configuration,
which is used instead if MTU of network is 0

Change-Id: I7c1140b116aa6304aca13479a7d7eb82d828d16f
Close-Bug: 1738812

kuryr_kubernetes/cni/binding/bridge.py

@@ -43,10 +43,18 @@ class BaseBridgeDriver(health.HealthHandler):
                 with h_ipdb.interfaces[host_ifname] as h_iface:
                     h_iface.remove()
 
+        if vif.network.mtu:
+            interface_mtu = vif.network.mtu
+        else:
+            LOG.info("Default mtu %(mtu)s is used for interface, "
+                     "for mtu of network if configured with 0",
+                     {"mtu": CONF.neutron_defaults.network_device_mtu})
+            interface_mtu = CONF.neutron_defaults.network_device_mtu
+
         with b_base.get_ipdb(netns) as c_ipdb:
             with c_ipdb.create(ifname=ifname, peer=host_ifname,
                                kind='veth') as c_iface:
-                c_iface.mtu = vif.network.mtu
+                c_iface.mtu = interface_mtu
                 c_iface.address = str(vif.address)
                 c_iface.up()
 
@@ -56,7 +64,7 @@ class BaseBridgeDriver(health.HealthHandler):
 
         with b_base.get_ipdb() as h_ipdb:
             with h_ipdb.interfaces[host_ifname] as h_iface:
-                h_iface.mtu = vif.network.mtu
+                h_iface.mtu = interface_mtu
                 h_iface.up()
 
     def disconnect(self, vif, ifname, netns):

kuryr_kubernetes/config.py

@@ -163,6 +163,9 @@ neutron_defaults = [
     cfg.StrOpt('external_svc_subnet',
                help=_("Optional external subnet ID for Kubernetes services"),
                default=None),
+    cfg.IntOpt('network_device_mtu',
+               help='Default MTU setting for network interface.',
+               default=1500,),
 ]
 
 octavia_defaults = [