Add support for service type "none" for linux VMs

[gscho]

Having a service type of none for linux VMs would be desirable so that users can opt to manage the chef-client process using one of the built in chef_client resources:

• chef_client_cron
• chef_client_launchd
• chef_client_systemd_timer

As a work around, a user must first remove the cron entry manually in their base cookbook:

cron  'azure_chef_extension' do
  action :delete
end

chef_client_systemd_timer 'chef-client' do
  ...
end