Fixes #224: Added ipv6 support to openwisp-get-address

[Bartosz-lab]

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 #224.

Description of Changes

• Added support for getting the IPv6 address if the IPv4 address is not found in openwisp-get-address.

[nemesifier]

Thanks for this. I have one doubt, what's the point of the --prefer-ipv6 flag if it's not used in the rest of the code?

[Bartosz-lab]

Currently, the --prefer-ipv6 flag is not used anywhere in the code, but in the future, we may introduce a configuration option that would allow users to choose this preference.
However, if we decide not to introduce this option, I will remove the flag.

[nemesifier]

Currently, the --prefer-ipv6 flag is not used anywhere in the code, but in the future, we may introduce a configuration option that would allow users to choose this preference. However, if we decide not to introduce this option, I will remove the flag.

As a general rule we add only code which is used. OpenWISP has quite a good number of modules which sums up as many lines of code, maintained by a very small group of people, so we need to make sure to add only what we really need. The problem is not just adding new lines of code but maintaining them over time.

[Bartosz-lab]

I have made the modifications to include only the necessary code.

[nemesifier]

Thanks @Bartosz-lab 🙏