groups added with a user list must have all users present.

$ cat my.yaml
#cloud-config
groups:
  - basic
  - group1: [foobar]
users:
  - name: foobar
    groups: basic

$ lxc launch ubuntu-daily:artful "--config=user.user-data=$(cat my.yaml)" a2
...
$ lxc exec a2 -- grep WARN /var/log/cloud-init.log

2017-10-18 17:18:27,034 - __init__.py[WARNING]: Unable to add group member 'foobar' to group 'group1'; user does not exist.

I found this bug when looking at intergration test WARNINGs per bug 1724354.

I'm not sure what the right change is, options include:
 * document this
 * move user creation before groups creation. This will probably work, since we now create groups by default that do not already exist when creating users.
 * run 2 passes, or otherwise read group content before adding groups.

Related bugs:
 * bug 1724354: WARNING in logs due to missing python-jsconschema

ProblemType: Bug
DistroRelease: Ubuntu 17.10
Package: cloud-init 17.1-18-gd4f70470-0ubuntu1
ProcVersionSignature: Ubuntu 4.13.0-15.16-generic 4.13.4
Uname: Linux 4.13.0-15-generic x86_64
ApportVersion: 2.20.7-0ubuntu3
Architecture: amd64
CloudName: None
Date: Wed Oct 18 17:18:30 2017
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: 2017-10-18 17:18:27,034 - __init__.py[WARNING]: Unable to add group member 'foobar' to group 'group1'; user does not exist.