ISSUE-645 move diagnostic user config out of tier configurations

[APegaDavis]

closes #645

The pega-diagnostic-secret is not tier-specific, so the current requirement that it is defined in a tier-specific way doesn't make sense. While configuring the diagnostic user on tiers that are not accessible through a k8s Service doesn't serve much purpose, it's also harmless.

This PR:

• Moves pegaDiagnosticUser and pegaDiagnosticPassword out of the tier-specific configuration and onto the global level
• Maintains support for existing configurations (tier-specific config for a tier named 'web')

[gitguardian[bot]]

⚠️ GitGuardian has uncovered 2 secrets following the scan of your pull request.

Please consider investigating the findings and remediating the incidents. Failure to do so may lead to compromising the associated services or software components.

🔎 Detected hardcoded secrets in your pull request

GitGuardian id	GitGuardian status	Secret	Commit	Filename
-		Generic Password	1cbf9972f377dca9ba84c7ef1ace4dd53729c84d	terratest/src/test/pega/pega-diagnostic-secret_test.go	View secret
-		Generic Password	1cbf9972f377dca9ba84c7ef1ace4dd53729c84d	terratest/src/test/pega/data/values_with_tier_diagnostic.yaml	View secret

🛠 Guidelines to remediate hardcoded secrets

1. Understand the implications of revoking this secret by investigating where it is used in your code.
2. Replace and store your secrets safely. Learn here the best practices.
3. Revoke and rotate these secrets.
4. If possible, rewrite git history. Rewriting git history is not a trivial act. You might completely break other contributing developers' workflow and you risk accidentally deleting legitimate data.

To avoid such incidents in the future consider

• following these best practices for managing and storing secrets including API keys and other credentials
• install secret detection on pre-commit to catch secret before it leaves your machine and ease remediation.

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