Allow un-managed reverse lookup zones

[wiktorn]

Allow creation of a lookup zones in Cloud DNS that have records (unmanaged). Add a flag to create managed zones, where records are added automatically.

I'm not sure what is the exact use case for managed reverse lookup zone, so I decided to make it a non-default option, and make an un-managed the default, happy to change that if managed approach is prevalent.

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Checklist

I applicable, I acknowledge that I have:

• Read the contributing guide
• Ran terraform fmt on all modified files
• Regenerated the relevant README.md files using tools/tfdoc.py
• Made sure all relevant tests pass

Breaking Changes

`modules/dns`: Reverse zones are by default created as unmanaged now. To keep your zone as managed, please set `var.zone_config.private.reverse_managed` to `true`