Enhance simple_command to close stdin to avoid this class of bugs in the future.

[xiangce]

          I'd suggest to enhance `simple_command` to close stdin to avoid this class of bugs in the future.

Originally posted by @ekohl in #4186 (review)

For more details see:

• Fixes #37720 - Execute insights registration with empty input theforeman/foreman#10267
• RHINENG-11883

[ShimShtein]

I think closing the stdin can also influence the caller process, which is undesirable.

Maybe the simple_command should use the equivalent of ruby's popen3:

Creates streams stdin, stdout, and stderr, which are the standard input, standard output, and standard error streams in the child process.

This way the child process would be completely isolated from the caller.

[ekohl]

The module already extensively uses subprocess which you can easily pass stdin=subprocess.DEVNULL to achieve that.