[fix] Fixed automatic attachment of wireless interface to network interface #314

[nemesifier]

Checklist

• I have read the OpenWISP Contributing Guidelines.
• I have manually tested the changes proposed in this pull request.
• I have written new test cases for new code and/or updated existing tests for changes to existing code.
• I have updated the documentation.

Reference to Existing Issue

Fixes #314.

Description of Changes

If the wireless interface has any address defined, attach it automatically to the related network interface.

[coveralls]

coverage: 99.499% (+0.002%) from 99.497%
when pulling d705b69 on issues/314-fix-network-device-syntax
into 5d8edf4 on master.