Add DockerPodman class

This allows us to set the beaker hypervisor not only to `docker`,
but also to `docker_podmand`. This enables us to implement podman
specific patches (if we ever need to). But it also makes it easier
for modules to specify if they want to use podman or docker. See
voxpupuli/gha-puppet#48 for reference.

lib/beaker/hypervisor/docker_podman.rb

@@ -0,0 +1,4 @@
+module Beaker
+  class DockerPodman < Beaker::Docker
+  end
+end