Allow the flannel backend to be specified as
backend=option=val,option2=val2 to select a given backend with extra options.

In particular this adds the following options to wireguard-native
backend:
* Mode - flannel wireguard tunnel mode
* PersistentKeepaliveInterval- wireguard persistent keepalive interval
Signed-off-by: Sjoerd Simons <sjoerd@collabora.com>