Define the message to send to spark room

Environment variable examples:

• ${BUILD_URL}
• ${BUILD_NUMBER}
• ${BUILD_ID}
• ${BUILD_DISPLAY_NAME}
• ${BUILD_TAG}
• ${JOB_NAME}
• ${JOB_URL}
• ${EXECUTOR_NUMBER}
• ${HUDSON_URL}
• ${JAVA_HOME}
• ${JAVA_VERSION}
• ${JENKINS_URL}
• ${JENKINS_VERSION}
• ${NODE_LABELS}
• ${NODE_NAME}
• ${PATH}
• ${PWD}

if using SCM or SCM trigger (see the corresponding plugin wiki for additional environment variables):

• ${BRANCH_NAME}
• ${CHANGE_ID}
• ${CHANGE_URL}
• ${CHANGE_TITLE}
• ${CHANGE_AUTHOR}
• ${CHANGE_AUTHOR_DISPLAY_NAME}
• ${CHANGE_AUTHOR_EMAIL}
• ${CHANGE_TARGET}

if using the Environment Injector Plugin:

• ${BUILD_CAUSE}
• ${BUILD_CAUSE_MANUALTRIGGER}
• ${BUILD_CAUSE_SCMTRIGGER}
• ${BUILD_CAUSE_UPSTREAMTRIGGER}
• ${ROOT_BUILD_CAUSE}

All environment variables injected into the build can also be used