The "payload" for the MQTT message.

In addition to environment variables and build parameters, the following variables can also be used (all variables must be encapsulated within "${}"):

• BUILD_RESULT - The result of the build (e.g. SUCCESS, FAILURE, ABORTED, etc.)
• PROJECT_URL - The job project's URL (e.g. "job/my-build")
• CULPRITS - Comma-separated list of the culprits responsible for the current build result
• BUILD_NUMBER - the build number