Preparation for gluon v2018.1; Set vxlan to false

In non multi domain setups, vxlan is mandatory.

Release notes cites:
--- snip ---
In single domain setups, the new mesh.vxlan option is mandatory. It
should be set to true in new meshes; existing setups should set it to
false to retain compatibility with older versions of Gluon.
--- snap ---

It seems that we do have to wait until all Nodes are upgraded to
v2018.1, so we set it to false by now.

site.conf

@@ -43,6 +43,9 @@
         },
     },
 
+    mesh = {
+        vxlan = false,
+    },
 
     next_node = {
         ip4 = '172.21.24.254',