Bug#931173: Also breaks official Openstack Debian

[Michael Anderson <michael.anderson@elegosoft.com>]

Using the Openstack Debian image 
(https://cloud.debian.org/cdimage/openstack/current-10/debian-10-openstack-amd64.qcow2) 
with the following cloud-config:

---- network-config ----
version: 1
config:
    - type: physical
      name: eth0
      mac_address: '4e:3c:c1:69:fe:cc'
      subnets:
      - type: static
        address: '10.110.10.45'
        netmask: '255.255.255.0'
        gateway: '10.110.10.1'
      - type: dhcp6
    - type: nameserver
      address:
      - '213.133.100.100'
      - '213.133.99.99'
      - '213.133.98.98'
-----------------------

---- /etc/network/interfaces ----

results in duplicate configurations for eth0:

root@foo2:/home/debian# cat $(find /etc/network/interfaces* -type f)
# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

# The loopback network interface
auto lo
iface lo inet loopback

# The normal eth0
allow-hotplug eth0
iface eth0 inet dhcp

# Additional interfaces, just in case we're using
# multiple networks
allow-hotplug eth1
iface eth1 inet dhcp

allow-hotplug eth2
iface eth2 inet dhcp

# Set this one last, so that cloud-init or user can
# override defaults.
source /etc/network/interfaces.d/*

--------

---- /etc/network/interfaces.d/50-cloud-init.cfg ----

# This file is generated from information provided by
# the datasource.  Changes to it will not persist across an instance.
# To disable cloud-init's network configuration capabilities, write a file
# /etc/cloud/cloud.cfg.d/99-disable-network-config.cfg with the following:
# network: {config: disabled}
auto lo
iface lo inet loopback
    dns-nameservers 213.133.100.100 213.133.99.99 213.133.98.98

auto eth0
iface eth0 inet static
    address 10.110.10.45/24
    gateway 10.110.10.1

# control-alias eth0
iface eth0 inet6 dhcp
--------

If I pass a dhcp configuration to cloud-config, the image fails to boot.

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