[DNM] Adding support for Private DNS Public Preview

[muwaqar]

This PR is a continuation of #5757 which accidentally got closed.

This PR adds support for creating/updating Private DNS zones for the public preview release. It is based on the Python SDK PR Azure/azure-sdk-for-net#4109 which in turn is based on Swagger version 2018-03-01-preview Azure/azure-rest-api-specs#2587.

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General Guidelines

• The PR has modified HISTORY.rst describing any customer-facing, functional changes. Note that this does not include changes only to help content. (see Modifying change log).

Command Guidelines

• Each command and parameter has a meaningful description.
• Each new command has a test.

(see Authoring Command Modules)

[promptws]

View a preview at https://prompt.ws/r/Azure/azure-cli/5763
This is an experimental preview for @microsoft users.

[muwaqar]

Please do NOT merge the PR until the relevant Swagger and Python SDK PRs have been merged.

[muwaqar]

@tjprescott : Please have a look at the PR, and if you don't have any comments, then please merge it at your earliest convenience.

[tjprescott]

LGTM