Add release note about update of mtu config value

This commit adds a release note announcing the update
of the mtu config value and the reason behind it.

Change-Id: Ieeea42a49e358f48f0d567d9450dc9b2dcc14173

releasenotes/notes/network-device-mtu-default-90faa7d685d4d0ec.yaml

@@ -0,0 +1,7 @@
+---
+upgrade:
+  - |
+    In order to match the default value of MTU defined
+    in Neutron, regardless of the SDN used, the default
+    value ``[neutron_defaults]network_device_mtu`` is now
+    set to 0.