Add subnet extension dhcp-mtu and configure it in option26 of the dhcp binding.
Also add this column to the nsxv_subnet_ext_attributes DB table.
This option will be available only from NSX version 6.2.3
DocImpact: Added dhcp-mtu extension to subnets
Change-Id: Id2a74a3c089beb61fde6b7c0fd02b207e444c3b7