[Subnet Prioritization] Add EnableSingleAvailabilityZone to Fleet Config

[Allenz5]

Description of changes

Add EnableSingleAvailabilityZone parameter to fleet_config.json

Tests

• test_fleet_config_generator.py

References

• Link to impacted open issues.
• Link to related PRs in other packages (i.e. cookbook, node).
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[himani2411]

We don't want to showcase this error if Customer doesn't provide this parameter as this is not required parameter.

[himani2411]

I understand this is a test case but we want to cover the case where we don't see this error and fleet-config.json gets created even if this parameter is not provided.

[himani2411]

As this fleet-config.json is what drives the scaling through node package