Add ability to provide fields to remove from Rollbar payload [CIRCLE-16074]

[tanya-coding]

This allows services to customize what PII information should not be logged to Rollbar.

This change is prompted by the following JIRA: https://circleci.atlassian.net/browse/CIRCLE-16074

Considerations:

• We think that it should be safe to remove fields outside of the try/catch block because walking payload map should never fail because it is looking for exact match of keys
• If we could extend JSON factory there might be a more efficient way to remove fields during JSON generation but we didn't find an easy way to extend the factory: https://github.com/dakrone/cheshire/blob/master/src/cheshire/generate.clj#L73
• Some function names are not ideal but we had a hard time coming up with something better so suggestions are welcome