[feature] Automatically create management VPN

[nemesifier]

This feature can be enabled by default, users should be able to disable it with a configuration variable, we should guide users to do this choice in the beginning of the tutorial.

If enabled, we should install openvpn, we should create a VPN server object (if no VPN server object exist yet), we should place a default working configuration for a layer 3 VPN (routed), then create a VPN-client template based on it and flag this template as default.
We should then take the openvpn server configuration just saved into OpenWISP and deploy it in the linux filesystem, enable the openvpn service and start it.

This way we'll get a basic management VPN working out of the box for new installations, which coupled with other features (#195 #199 #200) will make this role a lot more powerful and helpful for the community.