Update Envoy image version to v1.15.1.0-prod

[landesherr]

Issue: aws/aws-app-mesh-roadmap#268

Description of changes: Update the Envoy image version to the latest image, v1.15.1.0-prod
Additional Changes:

• Outlier detection event logging added to bootstrap config
• Bootstrap now supports "truthy" environment variables
• Bootstrap config found in non-bootstrap config files is now merged into the bootstrap

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[fawadkhaliq]

Thanks for the PR @landesherr

Adding Envoy v1.15.1 release notes for reference: https://www.envoyproxy.io/docs/envoy/latest/version_history/v1.15.1

In addition to the release notes, is there anything else changed in the bootstrap config (or otherwise) for the App Mesh packaged Envoy image? If yes, please consider adding a summary of changes in the PR description

[landesherr]

Hi @fawadkhaliq
I've updated the PR description with the changes to the bootstrap config that were rolled into the image release.

[fawadkhaliq]

Hi @fawadkhaliq
I've updated the PR description with the changes to the bootstrap config that were rolled into the image release.

++, thank you!!