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

tests/files/vagrant_centos8-kube-router.rb

@@ -0,0 +1,15 @@
+$num_instances = 2
+$vm_memory ||= 2048
+$os = "centos8"
+
+$kube_master_instances = 1
+$etcd_instances = 1
+
+# For CI we are not worried about data persistence across reboot
+$libvirt_volume_cache = "unsafe"
+
+# Checking for box update can trigger API rate limiting
+# https://www.vagrantup.com/docs/vagrant-cloud/request-limits.html
+$box_check_update = false
+
+$network_plugin = "kube-router"