Improve prepare_test_env
    
    - remove superfluous intermediary variables, redirect OS grains
      and roles output directly
    - apply ShellCheck suggestions:
      - quote variables and arrays to prevent globbing and word splitting
      - test array elements (arrays themselves do not work as operands in
        `test`)
      - remove unreferenced variables
    
    Signed-off-by: Georg Pfuetzenreuter <mail@georg-pfuetzenreuter.net>