[ch5479] Ensure pgo test shows instances in "unknown" state

[jkatz]

Checklist:

• Have you added an explanation of what your changes do and why you'd like them to be included?
• Have you updated or added documentation for the change, as applicable?
• Have you tested your changes on all related environments with successful results, as applicable?

Type of Changes:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

What is the current behavior? (link to any open issues here)

When a PostgreSQL instance is in an indeterminate state, e.g. during
initialization or when healing after a failover event, the type of
instance is unknown. However, pgo test, and by extension the test
endpoint in the apiserver, were not returning the status of the
instances that were in this state.

What is the new behavior (if this is a feature change)?

This patch corrects this by allowing instances to have a type of
"unknown".

Issue: [ch5479]