Bugfixes

Fixed hardcoded storage backend of the "FirmwareImage.file" field. FirmwareImage.file was configured to use PrivateFileSystemStorage, which made it impossible to use other private storage backends. The "OPENWISP_FIRMWARE_PRIVATE_STORAGE_INSTANCE" setting is added to make the FirmwareImage.file storage configurable.
Fixed inconsistent URL keyword argument in "serve_private_file" URL pattern. This broke the reverse proxy feature of django-private-storage.

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