BZ #1151439: Set MTU for VXLAN and GRE to -50 of MTU of physint

https://bugzilla.redhat.com/show_bug.cgi?id=1151439

Actually compute the MTU instead of just returning the given value.
Actually this should be:
+      this.enable_tunneling? ? this.network_device_mtu - 50 : this.network_device_mtu
instead of
+      this.ovs_tunnel_types.empty? ? this.network_device_mtu : this.network_device_mtu - 50

Relying on ovs_tunnel_types being empty or not is more of an impl detail, and enable_tunneling? is the explicit check as to whether this is VXLAN or GRE (note the latter two expressions are reversed, since enable_tunneling? is true when ovs_tunnel_types is *not* empty.