DataSourceAzure: fix bug where reported ready marker file is being written before report ready in preprovisioning

[johnsonshi]

Proposed Commit Message

DataSourceAzure: Write reported ready marker file after report ready in preprovisioning

DataSourceAzure previously writes the preprovisioning
reported ready marker file before it goes through the
report ready workflow. On certain VM instances, the
marker file is successfully written but then reporting
ready fails.

Upon rare VM reboots by the platform, cloud-init sees
that the report ready marker file already exists.
The existence of this marker file tells cloud-init
not to report ready again (because it mistakenly
assumes that it already reported ready in
preprovisioning).

In this scenario, cloud-init instead erroneously
takes the reprovisioning workflow instead of
reporting ready again.

Additional Context

Additional context is already in the proposed commit message.

Test Steps

• Create an Azure VM from the latest Ubuntu 18.04 LTS image.
• Apply this PR's patch.
• Capture the Azure VM into a custom image.
• Successful deployment from this captured custom image.

Checklist:

• My code follows the process laid out in the documentation
• I have updated or added any unit tests accordingly
• I have updated or added any documentation accordingly

[johnsonshi]

@anhvoms for awareness

[OddBloke]

This looks good, thanks! A few style/logging Qs inline.

[OddBloke]

Thanks Johnson, this looks good to me. I particularly appreciate the clear and complete testing code, it made it much easier to determine whether or not you had modified behaviour intentionally!