Allow specifying chef omnibus download version

$ cat chef.yml
#cloud-config
chef:
  install_type: "omnibus"
  node_name: "your-node-name"
  server_url: "https://bogus.host"
  validation_name: "https://bogus.host"
  validation_cert: "system"
  omnibus_version: "12.3.0"   # check for this version


$ cat 1462693.sh
#!/bin/bash

for release in xenial zesty;
  do
    ref=$release-proposed;
    name=test-$release;
    lxc-proposed-snapshot --proposed --publish $release $ref;
    lxc init $ref $name;
    lxc config set $name user.user-data - < chef.yml;
    lxc start $name;
    sleep 30; # For chef download/install/apply
    lxc exec $name -- grep 'Unpacking chef' /var/log/cloud-init-output.log;  # expect to see version 12.3.0-X
    lxc exec $name -- dpkg-query show cloud-init;
  done


$ ./1462693.sh 2>&1 | tee 1462693.log

--- output from script---
Creating xenial-proposed-1592712276
--proposed -- 
...
Get:1 http://archive.ubuntu.com/ubuntu xenial-proposed/main amd64 cloud-init all 17.1-18-gd4f70470-0ubuntu1~16.04.1 [342 kB]
Preconfiguring packages ...
Creating test-xenial


Unpacking chef (12.3.0-1) ...
cloud-init	17.1-18-gd4f70470-0ubuntu1~16.04.1

Creating zesty-proposed-2554229982
--proposed -- 
Get:1 http://archive.ubuntu.com/ubuntu zesty-proposed/main amd64 cloud-init all 17.1-18-gd4f70470-0ubuntu1~17.04.1 [340 kB]
Preconfiguring packages ...
Creating test-zesty


Unpacking chef (12.3.0-1) ...
cloud-init	17.1-18-gd4f70470-0ubuntu1~17.04.1