[feature] Allow defining Wireguard peers in the server to reserve IP addresses

[nemesifier]

Right now to allow non OpenWISP peers to connect to a Wireguard server managed by OpenWISP, the user needs to create fake devices which act as placeholders and reserve the IP addresses needed by these clients.

We could make this easier in the following way:

• when peers are defined in the server configuration automatically create the IP address entries in IPAM
• we have to find a way to remove the IPAM entries if the peers are removed
• we need to make sure peers defined in the server config gets passed to the resulting configuration installed on the server