CI/molecule: use common vars as well and simplify run (#13520)

The molecule_run.sh isn't really needed since the switch to gitlab-ci
matrix runs.

Also use the inventory variables pattern for kata-containers.

roles/container-engine/kata-containers/molecule/default/converge.yml

@@ -2,9 +2,6 @@
 - name: Converge
   hosts: all
   become: true
-  vars:
-    kata_containers_enabled: true
-    container_manager: containerd
   roles:
     - role: kubespray_defaults
     - role: container-engine/containerd