Paunch warning are logged on standard output, cannot be silenced

In recent podman-based environment such as those used by undercloud upgrade CI job [1], when I run paunch to list the configured containers on the system, I get spurious warnings:

[zuul@subnode-0 ~]$ sudo paunch list --default-runtime docker -f json
image_exist isn't supported by docker
image_exist isn't supported by docker
image_exist isn't supported by docker
image_exist isn't supported by docker
image_exist isn't supported by docker
image_exist isn't supported by docker
[...]
[
  {
    "status": "running",
    "container": "rabbitmq",
    "image": "docker.io/tripleorocky/centos-binary-rabbitmq:50a020cb9e7ad4031b4023cc1c881afab83ce889_fbb4de96",
    "config": "tripleo_step1",
    "command": "kolla_start"
  },
[...]

The problem is that those warning cannot be silenced because paunch logs warnings both logs in the log file (can be overriden at command line) _and_ to stdout [2].

Consequently, paunch's output is mangled with warning and this makes it hard to parse and process.

[1] e.g. tripleo-ci-centos-7-containerized-undercloud-upgrades
[2] https://github.com/openstack/paunch/blob/master/paunch/utils/common.py#L36