pacemaker: force the use of docker for ContainerCli

Until we get CentOS8 to test Pacemaker with Podman, we need to keep
using Docker for ContainerCli now.

Later, when CentOS8 is out, we'll have a conditional we only set this
variable to 'docker' on CentOS7.

Change-Id: I7fc9a880f15463fd7d95be5ae46c1c1d2f339448

config/general_config/pacemaker.yml

@@ -98,3 +98,6 @@ deploy_steps_ansible_workflow: >-
   {%- else -%}
   false
   {%- endif -%}
+
+# Switch this variable to 'podman' once we run OVB jobs on CentOS8.
+overcloud_container_cli: docker