Set stdout so CLI completion feature works correctly

[lynnemorrison]

fixes 2135

[fgiorgetti]

I am not sure on the benefit of changing exclusively the output of skupper completion to stderr.

If I understand the original issue correctly, the problem reported says that on an invalid usage, the help message is written to stdout, which is where the shell completion code is being expected.

Maybe instead of just skupper completion going to stderr, if an invalid param is also passed to the completion command, like: skupper completion slash, the error/usage should go to stderr.

Thoughts?