containerd,docker: stop installing extras repo on CentOS/RHEL (#7203)

This was introduced in 143e2272ff
Extra repo is enabled by default in CentOS, and is not the right repo for EL8
Instead of adding a CentOS repo to RHEL, enable the needed RHEL repos with rhsm_repository

For RHEL 7, we need the "extras" repo for container-selinux
For RHEL 8, we need the "appstream" repo for container-selinux, ipvsadm and socat

Signed-off-by: Etienne Champetier <champetier.etienne@gmail.com>

inventory/sample/group_vars/k8s-cluster/offline.yml

@@ -34,12 +34,12 @@
 # calicoctl_download_url: "{{ files_repo }}/kubernetes/calico/{{ calico_ctl_version }}/calicoctl-linux-{{ image_arch }}"
 
 ## CentOS/Redhat
-### Docker
+### For EL7, base and extras repo must be available, for EL8, baseos and appstream
+### By default we enable those repo automatically
+# rhel_enable_repos: false
+### Docker / Containerd
 # docker_rh_repo_base_url: "{{ yum_repo }}/docker-ce/$releasever/$basearch"
 # docker_rh_repo_gpgkey: "{{ yum_repo }}/docker-ce/gpg"
-### Containerd
-# extras_rh_repo_base_url: "{{ yum_repo }}/centos/$releasever/extras/$basearch"
-# extras_rh_repo_gpgkey: "{{ yum_repo }}/containerd/gpg"
 
 ## Fedora
 ### Docker