malformed /etc/network/interfaces

The "compute-node" role in the "maas" repository writes out /etc/network/interfaces on all nodes. It uses the existing /etc/network/interfaces config file as input to the process. On the Flex1 pod we start with a config with:

auto ens3d1
iface ens3d1 inet manual
    mtu 1500

source /etc/network/interfaces.d/*.cfg

The new config file contains:

auto ens3d1
iface ens3d1 inet manual
  source /etc/network/interfaces.d/*.cfg
  mtu 1500

... which is invalid. The node will not come online after the new config is written.