[feature] Add support for openwisp-firmware-upgrader

[nemesifier]

We can install this module by default and enable its API, we can provide a variable to disable the module and another variable to disable only the API if needed.

[pandafy]

@nemesisdesign I don't see a potential use for creating a dedicated variable for disabling just APIs. The users can simple enable or disable APIs using by setting OPENWISP_FIRMWARE_UPGRADER_API in openwisp2_extra_django_settings variable.

[nemesifier]

@nemesisdesign I don't see a potential use for creating a dedicated variable for disabling just APIs. The users can simple enable or disable APIs using by setting OPENWISP_FIRMWARE_UPGRADER_API in openwisp2_extra_django_settings variable.

Agreed, what about a brief section for the firmware upgrader module in the README like we have for network topology?
In that section we could advise this (that API can be disabled this way, as well as setting other settings and link to the firmware upgrader settings).

[pandafy]

I have added it in the PR 😄