Need support for "complete --output" or similar to avoid clutter in stdout

The problem is that "complete" only supports outputting the bash completion code to stdout.

But what if along the way other libraries also emit to stdout? The result would be unusable.

This exact issue is happening right now with python-tripleoclient. See this issue:

https://bugzilla.redhat.com/show_bug.cgi?id=1672394

This could be fixed if we could do "openstack complete --output myfile.sh", which should really be the *only* correct way to do this.

I am also classifying this as a security vulnerability, because this could allow for malicious Python code to inject bash commands into the bash completion code.