fixed mtu resolution if not defined in config file
fixed mtu resolution if not defined in config file

Signed-off-by: Vladimir Lavor <0b39dc9488caacdbde86cd77d448fb8e74406983@cisco.com>

ondrej-fabry/vpp-agent,VladoLavor/vpp-agent,ondrej-fabry/vpp-agent,VladoLavor/vpp-agent,VladoLavor/vpp-agent,ondrej-fabry/vpp-agent