[gsoc21] Setting management IP: validation and interactions with other parts

Follow up to: https://github.com/openwisp/openwisp-controller/pull/496#pullrequestreview-730140216.

There are mainly two possible ways in which OpenWISP can be used:

A) every organization has its own management tunnel
B) all organizations share the same management tunnel

In case A) we have to ensure there cannot be 2 devices having the same management IP in an organization.
In case B) we have to ensure there cannot be 2 devices having the same management IP in the whole system.

The error message can be _('this management IP is already used by another device').

This behavior can be controlled with a setting (which has to be documented), which can default to use case B) which is the most common case for most users having small systems.

There's another important thing to take into account: the controller views already manage the management IP automatically, see the code below:

https://github.com/openwisp/openwisp-controller/blob/cd55ea427fedffffdc1e103bae948ab1b99a68da/openwisp_controller/config/controller/views.py#L73-L108

https://github.com/openwisp/openwisp-controller/blob/cd55ea427fedffffdc1e103bae948ab1b99a68da/openwisp_controller/config/utils.py#L64-L81

It looks to me that adding this validation would break that code. Maybe we can change the code to remove the duplicate before saving the new management IP to the device?