Add role for wireguard

Setup wireguard interfaces, configure and deploy keys.
Private keys are stored in a file on the host.
Public keys are stored in facts, enable fact-caching to allow running
the role on a subset of the peers.

The dictionary key (interface name) must be the same on all hosts (it
identifies the network). At least one peer needs to have a static port
specified, so other peers can connect to it.

Peers are specified using an inventory hostpattern.

This doesn't setup any packet forwarding, DNS server or other VPN
features. It just enabled p2p connections (to potentially multiple
peers), e.g. to allow serving usually unencrypted applications (like
NFS) over an encrypted connection.