Rename field so that GCP stackdriver logs are captured at the right level

[alexionescu]

Description

This PR will have the json log formatter in diagd.py have its levelname field renamed to severity so that GCP stackdriver logs are properly annotated with the right severity.

Related Issues

List related issues.

Testing

A few sentences describing what testing you've done, e.g., manual tests, automated tests, deployed in production, etc.

Checklist

• Does my change need to be backported to a previous release?

  • What backport versions were discussed with the Maintainers in the Issue?

• I made sure to update CHANGELOG.md.

  Remember, the CHANGELOG needs to mention:

  • Any new features
  • Any changes to our included version of Envoy
  • Any non-backward-compatible changes
  • Any deprecations

• This is unlikely to impact how Ambassador performs at scale.

  Remember, things that might have an impact at scale include:

  • Any significant changes in memory use that might require adjusting the memory limits
  • Any significant changes in CPU use that might require adjusting the CPU limits
  • Anything that might change how many replicas users should use
  • Changes that impact data-plane latency/scalability

• My change is adequately tested.

  Remember when considering testing:

  • Your change needs to be specifically covered by tests.
    • Tests need to cover all the states where your change is relevant: for example, if you add a behavior that can be enabled or disabled, you'll need tests that cover the enabled case and tests that cover the disabled case. It's not sufficient just to test with the behavior enabled.
  • You also need to make sure that the entire area being changed has adequate test coverage.
    • If existing tests don't actually cover the entire area being changed, add tests.
    • This applies even for aspects of the area that you're not changing – check the test coverage, and improve it if needed!
  • We should lean on the bulk of code being covered by unit tests, but...
  • ... an end-to-end test should cover the integration points

• I updated DEVELOPING.md with any any special dev tricks I had to use to work on this code efficiently.

• The changes in this PR have been reviewed for security concerns and adherence to security best practices.

[LanceEa]

@alexionescu - Thanks for this PR, would you mind writing up an issue first and/or finding an existing issue so that we can capture discussion and reasoning for a fix.

The key thing in this one is to think about whether it will have breaking changes on other users (maybe not but something to keep in mind).

[alexionescu]

@LanceEa Done, thank you for your feedback!