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

roles/container-engine/docker/meta/main.yml

@@ -1,5 +1,3 @@
 ---
 dependencies:
   - role: container-engine/containerd-common
-  - role: container-engine/docker-storage
-    when: docker_container_storage_setup and ansible_os_family == "RedHat"

roles/container-engine/docker/templates/docker.service.j2

@@ -1,19 +1,11 @@
 [Unit]
 Description=Docker Application Container Engine
 Documentation=http://docs.docker.com
-{% if ansible_os_family == "RedHat" %}
-After=network.target {{ ' docker-storage-setup.service' if docker_container_storage_setup else '' }} containerd.service
+After=network.target docker.socket containerd.service lvm2-monitor.service SuSEfirewall2.service
+{% if ansible_os_family != "Suse" %}
 BindsTo=containerd.service
-{{ 'Wants=docker-storage-setup.service' if docker_container_storage_setup else '' }}
-{% elif ansible_os_family == "Debian" %}
-After=network.target docker.socket containerd.service
-BindsTo=containerd.service
-Wants=docker.socket
-{% elif ansible_os_family == "Suse" %}
-After=network.target lvm2-monitor.service SuSEfirewall2.service
-# After=network.target containerd.service
-# BindsTo=containerd.service
 {% endif %}
+Wants=docker.socket
 
 [Service]
 Type=notify