allow nested config env override

WP [#17225](https://community.openproject.org/work_packages/17225)
Proposal:

<pre>
# create hash from path
env_var_hash = { path.last:  value}
path[0..-1].reverse.each do |key|
  env_var_hash[key] = env_var_hash
end
</pre>

and then merge this with the org_config.