Support configurable interface MTUs

MTUs were hardcoded at 9000, which breaks if the underlying interface
or network switch does not support jumbo frames, a possible deployment
limitation. This has non-obvious consequences due to MTU mismatches
for certain services (Ceph, Zookeeper, etc.).

This commit adds support for configurable MTUs for each interface,
set in pvcd.yaml. The example has been updated to reflect this, with
a default of 1500 (the Ethernet standard).

This commit also adds autoconfiguration of the VNI device MTU based
on the `vni_mtu` value, the same for bridge networks and minus 50
(rather than 200 from the hardcoded value, based on the following
resource [1]) for VXLAN networks.

[1] http://ipengineer.net/2014/06/vxlan-mtu-vs-ip-mtu-consideration/

node-daemon/pvcd/VXNetworkInstance.py

@@ -437,14 +437,18 @@ add rule inet filter forward ip6 saddr {netaddr6} counter jump {vxlannic}-out
                 self.vlan_nic
             )
         )
+
+        vx_mtu = self.config.vni_mtu
         common.run_os_command(
-            'ip link set {} mtu 8800 up'.format(
-                self.vlan_nic
+            'ip link set {} mtu {} up'.format(
+                self.vlan_nic,
+                vx_mtu
             )
         )
         common.run_os_command(
-            'ip link set {} mtu 8800 up'.format(
-                self.bridge_nic
+            'ip link set {} mtu {} up'.format(
+                self.bridge_nic,
+                vx_mtu
             )
         )
         common.run_os_command(
@@ -481,14 +485,18 @@ add rule inet filter forward ip6 saddr {netaddr6} counter jump {vxlannic}-out
                 self.vxlan_nic
             )
         )
+
+        vx_mtu = self.config.vni_mtu - 50
         common.run_os_command(
-            'ip link set {} mtu 8800 up'.format(
-                self.vxlan_nic
+            'ip link set {} mtu {} up'.format(
+                self.vxlan_nic,
+                vx_mtu
             )
         )
         common.run_os_command(
-            'ip link set {} mtu 8800 up'.format(
-                self.bridge_nic
+            'ip link set {} mtu {} up'.format(
+                self.bridge_nic,
+                vx_mtu
             )
         )
         common.run_os_command(