Add support for Kata Containers (#6256)

* Install Kata Containers as additional container runtime

* Create RuntimeClasses for Kata Containers

* Updated Vagrant to optionally run without Docker as container manager

* Updated Vagrant to optionally use Libvirt nested virtualization

* Add Kata Containers documentation

* Fix lint errors

* Add kata_containers_enabled to kubespray-defaults

* Fixed typo error

* Fixed typo error

Vagrantfile

@@ -55,6 +55,8 @@ $os ||= "ubuntu1804"
 $network_plugin ||= "flannel"
 # Setting multi_networking to true will install Multus: https://github.com/intel/multus-cni
 $multi_networking ||= false
+$download_run_once ||= "True"
+$download_force_cache ||= "True"
 # The first three nodes are etcd servers
 $etcd_instances ||= $num_instances
 # The first two nodes are kube masters
@@ -69,6 +71,7 @@ $override_disk_size ||= false
 $disk_size ||= "20GB"
 $local_path_provisioner_enabled ||= false
 $local_path_provisioner_claim_root ||= "/opt/local-path-provisioner/"
+$libvirt_nested ||= false
 
 $playbook ||= "cluster.yml"
 
@@ -146,6 +149,8 @@ Vagrant.configure("2") do |config|
       end
 
       node.vm.provider :libvirt do |lv|
+        lv.nested = $libvirt_nested
+        lv.cpu_mode = "host-model"
         lv.memory = $vm_memory
         lv.cpus = $vm_cpus
         lv.default_prefix = 'kubespray'
@@ -196,11 +201,11 @@ Vagrant.configure("2") do |config|
         "flannel_interface": "eth1",
         "kube_network_plugin": $network_plugin,
         "kube_network_plugin_multus": $multi_networking,
-        "download_run_once": "True",
+        "download_run_once": $download_run_once,
         "download_localhost": "False",
         "download_cache_dir": ENV['HOME'] + "/kubespray_cache",
         # Make kubespray cache even when download_run_once is false
-        "download_force_cache": "True",
+        "download_force_cache": $download_force_cache,
         # Keeping the cache on the nodes can improve provisioning speed while debugging kubespray
         "download_keep_remote_cache": "False",
         "docker_rpm_keepcache": "1",