Drop support for RHEL 7 / CentOS 7 (#11246)

* Simplify docker systemd unit

systemd handles missing unit by ignoring the dependency so we don't need
to template them.

* Remove RHEL 7/CentOS 7 support

- remove ref in kubespray roles
- move CI from centos 7 to 8
- remove docs related to centos7

* Remove container-storage-setup

Only used for RHEL 7 and CentOS 7

docs/developers/vagrant.md

@@ -80,7 +80,7 @@ cat << EOF > vagrant/config.rb
 \$instance_name_prefix = "kub"
 \$vm_cpus = 1
 \$num_instances = 3
-\$os = "centos-bento"
+\$os = "centos8-bento"
 \$subnet = "10.0.20"
 \$network_plugin = "flannel"
 \$inventory = "$INV"