[stress testing] Enable nodepool update via bicep and update node SKUs

[benbp]

• Updating stress cluster node SKUs - use d4v4 everywhere, separate system from user pools, and add stress-watcher to the system pool
• Enable updating stress cluster nodepools via bicep deployment via an -UpdateNodes switch.
• Remove warnings from bicep build
• Update documentation for pool SKU label selectors
• Minor provision script fixes: missing object id and custom environment parameter validation

Fixes #4830

[azure-sdk]

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[weshaggard]

looks reasonable

[ghost]

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