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

This was introduced in 143e2272
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>