Fixing VMSS Re-Image 7-Day Timer

[nharper285]

Summary of the Pull Request

What is this about?
The 7-Day VMSS Re-Image timer currently does not work. It relies upon the node 'TimeStamp' entry which is actually updated every few minutes when new node heartbeats come in. I am creating a new table entry - created_at - to fix this behavior.

PR Checklist

• Applies to work item: #xxx
• CLA signed. If not, go over here and sign the CLI.
• Tests added/passed
• Requires documentation to be updated
• I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan. Issue number where discussion took place: #xxx

Info on Pull Request

What does this include?

Validation Steps Performed

How does someone test & validate?

• Set timer to 15 min and observe timer reimage
• Deploy old instance with scaleset (w/o init_at field). Then deploy update over old instance and confirm scaleset updates with new field.

[chkeita]

@nharper285 can you update the description with your validation steps