* 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

* Add CI_TEST_VARS to tests

* Update flannel to 0.12.0 (with new manifests) and disable tx/rx
offloading in networking test

* Updates Controller Manager/Kubelet with Flannel's required configuration for CNI
* Removes old Flannel installation
* Install CNI enabled Flannel DaemonSet/ConfigMap/CNI bins and config (with portmap plugin) on host
* Uses RBAC if enabled
* Fixed an issue that could occur if br_netfilter is not a module and net.bridge.bridge-nf-call-iptables sysctl was not set