kops -h should show ENV variables

Our ENV variables are hidden from the user. I would recommend having another section in the help that describes which ENV variables we have and how to use them.

Start by locating the code that assembles the kops -h output and the definitions or usage references for its environment variables. Add a help section describing the available variables and how to use them, then verify that kops -h displays the section correctly.

Written by the indexing model from the issue text.