Feature/add flannel wireguard encryption backend as option (#9583)

* feat(): Add wireguard backend to flannel cni

As described in the flannel docs:
https://github.com/flannel-io/flannel/blob/master/Documentation/backends.md#wireguard

This does not support optional configuration methods like:
- setting a psk (will be autogenerated by default)
- chang listening ports
- change mode (defaults to 'separate')
- change PersistentKeepaliveInterval (defaults to 0)

* Add supported backends to flannel docs

* Fix markdown in docs

inventory/sample/group_vars/k8s_cluster/k8s-net-flannel.yml

@@ -10,8 +10,7 @@
 ## single quote and escape backslashes
 # flannel_interface_regexp: '10\\.0\\.[0-2]\\.\\d{1,3}'
 
-# You can choose what type of flannel backend to use: 'vxlan' or 'host-gw'
-# for experimental backend
+# You can choose what type of flannel backend to use: 'vxlan',  'host-gw' or 'wireguard'
 # please refer to flannel's docs : https://github.com/coreos/flannel/blob/master/README.md
 # flannel_backend_type: "vxlan"
 # flannel_vxlan_vni: 1