Ideally we'd have fully fleshed out support for it (i.e. #5011), but
that's a potentially breaking change and taking a little while to merge.

This is a much simpler change which won't break anything, but will allow
a "Type": "wireguard" reference in the "--flannel-conf" custom config
file to work.
Signed-off-by: Euan Kemp <euank@euank.com>