[change] Allow determining the management_ip automatically

[nemesifier]

We can determine the management IP from the configuration we download from OpenWISP, instead of having the user have to define this.

This will probably need a change in the server (eg: we can allow flagging a template as a management interface and allow specifying the interface name.

Once the agent sets the new management interface, it will have to report the IP (which could be assigned dynamically as in the case of OpenVPN) to the server.