agent: make Ambassador Agent opt-in

[LanceEa]

Description

The Ambassador Agent is not a necessary component to run emissary-ingress as an API Gateway. This removes it from the yaml manifest published and turns it off by default for the Helm chart published.

We recommend you switch to the stand alone chart which can be found here https://github.com/datawire/ambassador-agent.

Related Issues

N/A - cleanup

Testing

CI is green

Checklist

• [ ] Does my change need to be backported to a previous release?
• [x] I made sure to update CHANGELOG.md.
• [x] This is unlikely to impact how Ambassador performs at scale.
• [x] My change is adequately tested.
• [ ] I updated DEVELOPING.md with any any special dev tricks I had to use to work on this code efficiently.
• [x] The changes in this PR have been reviewed for security concerns and adherence to security best practices.