Well, actualy using ENV in such way https://github.com/crowbar/barclamp-quantum/blob/3a5e807fb002acf413e97059cb46891bc5491388/chef/cookbooks/quantum/recipes/post_install_conf.rb#L75 is kinda bad idea.
There is 2 phase of chef run - in first phase ruby code executed, on second phase the chefs dsl resources executed as well. If there is one more ENV['OS_USERNAME'] call in ruby code somewhere will be executed then when on second phase this execute block will be run https://github.com/crowbar/barclamp-quantum/blob/3a5e807fb002acf413e97059cb46891bc5491388/chef/cookbooks/quantum/recipes/post_install_conf.rb#L95 the OS_USERNAME will contain latest username.
So i'll keep current behavior for now, and maybe later i'll wrap both ENV call into ruby_block or something.