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

@@ -1,6 +1,6 @@
 ---
 # Instance settings
-cloud_image: centos-7
+cloud_image: centos-8
 mode: ha
 
 # Kubespray settings

tests/files/packet_centos8-calico-ha.yml

@@ -1,6 +1,6 @@
 ---
 # Instance settings
-cloud_image: centos-7
+cloud_image: centos-8
 mode: ha
 
 # Kubespray settings

tests/files/packet_centos8-flannel-addons-ha.yml

@@ -1,6 +1,6 @@
 ---
 # Instance settings
-cloud_image: centos-7
+cloud_image: centos-8
 mode: ha
 
 # Kubespray settings

tests/files/packet_centos8-multus-calico.yml

@@ -1,6 +1,6 @@
 ---
 # Instance settings
-cloud_image: centos-7
+cloud_image: centos-8
 mode: default
 
 # Kubespray settings

tests/files/packet_centos8-weave-upgrade-ha.yml

@@ -1,6 +1,6 @@
 ---
 # Instance settings
-cloud_image: centos-7
+cloud_image: centos-8
 mode: ha
 
 # Kubespray settings

tests/files/vagrant_centos8-kube-router.rb

@@ -1,6 +1,6 @@
 $num_instances = 2
 $vm_memory ||= 2048
-$os = "centos"
+$os = "centos8"
 
 $kube_master_instances = 1
 $etcd_instances = 1

tests/files/vagrant_centos8-kube-router.yml

@@ -1,6 +1,6 @@
 ---
 # Instance settings
-cloud_image: centos-7
+cloud_image: centos-8
 mode: default
 
 # Kubespray settings