Added wireguard_always_listen to force ListenPort without setting an endpoint

[alfvar]

I had a niche use case where I wanted to configure a client/server setup with two machines as the server on a shared virtual ip. I did not want mesh behavior so I did not want to define a wireguard_endpoint, but ListenPort would not propagate into the config of the target machines.

• Introduce new boolean var wireguard_always_listen (default false)
• Retain original behavior: only render ListenPort when an endpoint is defined or single-node
• Allow VIP or client/server setups to explicitly include ListenPort without setting an endpoint

[lae]

should probably be considered a fix for #214

(and I ran into this environment-crippling issue as well after being forced to update from 17.0.0)

[rdbisme]

I have the same problem (which broke my previous configuration when I updated the role).

Can we merge this?