juju sends "network" top level key to user.network-config in lxd containers

== Short summary ==
In lxd containers launched by juju,
/var/lib/cloud/seed/nocloud-net/network-config has:
 has:
 network:
   config: disabled

That is invalid content. Cloud-init assumes content in 'network-config'
is already namespaced to 'network'. The correct content would be:

  config: disabled

== Easy recreate ==
$ lxc launch ubuntu-daily:bionic \
   "--config=user.network-config={'network': {'config': {'disabled'}}}"

== Longer Info ==
When looking at bug 1651497, I see containers that run cloud-init
have errors in a container's cloud-init log
(http://paste.ubuntu.com/p/5mKXC8pMwH/) like:
  AttributeError: 'NoneType' object has no attribute 'iter_interfaces'
and
  Failed to rename devices: Failed to apply network config names. Found bad network config version: None

After some looking guessing I realized that juju must be attempting to
disable cloud-init's network configuration via sending the following
into the nocloud seed (/var/lib/cloud/seed/nocloud-net/network-config)
via 'user.network-config'.

cloud-init can clearly handle this better, but juju should not be
sending invalid configuration.

Related bugs:
 * bug 1651497: iscsid.service fails to start in container, results in failed dist-upgrade later on
 * bug 1906187: Version tag is not respected when put last

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: cloud-init 18.3-9-g2e62cb8a-0ubuntu1~18.04.2
ProcVersionSignature: Ubuntu 4.18.0-8.9-generic 4.18.7
Uname: Linux 4.18.0-8-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.4
Architecture: amd64
CloudName: NoCloud
Date: Tue Oct 16 14:33:12 2018
PackageArchitecture: all
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 LANG=C.UTF-8
SourcePackage: cloud-init
UpgradeStatus: No upgrade log present (probably fresh install)
cloud-init-log-warnings:
 2018-10-16 14:32:01,706 - stages.py[WARNING]: Failed to rename devices: Failed to apply network config names. Found bad network config version: None
 2018-10-16 14:32:01,707 - util.py[WARNING]: failed stage init-local
 AttributeError: 'NoneType' object has no attribute 'version'
 2018-10-16 14:32:02,366 - stages.py[WARNING]: Failed to rename devices: Failed to apply network config names. Found bad network config version: None
user_data.txt:
 #cloud-config
 {}