overcloud-deploy: introduce overcloud_container_cli parameter

overcloud_container_cli is a new parameter that will be used by the
featuresets where podman is enabled on the overcloud.
The default is now set to 'docker' for backward compatibility but in
Stein we'll switch the default to Podman.

Change-Id: I34813b240284c8d9b35e044bc9654d556ec0cbdb

roles/overcloud-deploy/defaults/main.yml

@@ -170,3 +170,7 @@ resource_registry_args:
   OS::TripleO::Controller::Net::SoftwareConfig: nic-configs/controller.yaml
   OS::TripleO::ObjectStorage::Net::SoftwareConfig: nic-configs/swift-storage.yaml
   OS::TripleO::CephStorage::Net::SoftwareConfig: nic-configs/ceph-storage.yaml
+
+# Used to deploy podman on the undercloud instead of docker
+# TODO(emilien) in Stein, we'll switch the default to podman
+overcloud_container_cli: docker

roles/overcloud-deploy/tasks/pre-deploy.yml

@@ -195,6 +195,9 @@
       {% else %}
       -e {{ working_dir }}/containers-prepare-parameter.yaml
       {% endif %}
+      {% if release not in ['pike', 'queens', 'rocky'] and overcloud_container_cli == 'podman' %}
+      -e {{ overcloud_templates_path }}/environments/podman.yaml
+      {% endif %}
   when:
     - containerized_overcloud|bool