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/packet_centos8-calico-ha-once-localhost.yml

@@ -0,0 +1,18 @@
+---
+# Instance settings
+cloud_image: centos-8
+mode: ha
+
+# Kubespray settings
+download_localhost: true
+download_run_once: true
+typha_enabled: true
+calico_apiserver_enabled: true
+calico_backend: kdd
+typha_secure: true
+disable_ipv6_dns: true
+auto_renew_certificates: true
+
+# Docker settings
+container_manager: docker
+etcd_deployment_type: docker