T4898: Add mtu config option for dummy interfaces

I use dummy interfaces in a VRF as source-interfaces for VXLAN in order to
force VXLAN send underlay UDP traffic through the VRF where the dummy interface
resides.

However dummy interface has no mtu option so it always gets an MTU of 1500.
This will cause an error when the mtu of dummy is not large enough for the
VXLAN traffic.

Adding this option in the config template will solve this.

(cherry picked from commit 1440ef93e13d15e2247cbfc2cb4ea2afb266fc9e)