It should be possible to configure the DHCP server to
provide the mtu option for inspection subnets.
This uses the dhcp-option-force so that the option is
provided also when the client does not ask for it.
According to dnsmasq manual page this is sometimes
required when sending options to PXELINUX.
Closes-Bug: #1845487
Change-Id: Ic95dbf1867fb5397f1b6d8f23466910a97051cb6