Add podman_docker support

This adds support for a `docker_podman` hypervisor. it will call the
default docker implementation at the moment. See voxpupuli/gha-puppet#48
for context.

lib/beaker-hostgenerator/hypervisor.rb

@@ -43,6 +43,7 @@ def self.builtin_hypervisors
         'vagrant' => BeakerHostGenerator::Hypervisor::Vagrant,
         'vagrant_libvirt' => BeakerHostGenerator::Hypervisor::Vagrant,
         'docker' => BeakerHostGenerator::Hypervisor::Docker,
+        'docker_podman' => BeakerHostGenerator::Hypervisor::Docker,
         'abs' => BeakerHostGenerator::Hypervisor::ABS,
         'hcloud' => BeakerHostGenerator::Hypervisor::Hcloud,
       }