Azure DNS CAA record support

[ranieuwe]

This PR Fixes #890

Steps taken:

• Upgraded vendor.json to include new DNS API from 2017-10-01
• Upgraded API for all other DNS providers
• Implemented the CAA provider based of the SRV provider
• Set up tests for the CAA provider
• Ran tests for DNS and CAA provider and they are all ✔️
• Documentation

All DNS tests have been executed (testacc, test) and are successful.

Introduction of the 2017-10-01 API does also allow for extended ASCII sets for TXT records. Have not tested or implemented that yet.

First contribution to Terraform. Looking forward to your review!

[WodansSon]

Hey @ranieuwe,

Thanks for the PR and congrats on your first contribution to Terraform. After review it LGTM.

[ghost]

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