[ENV-451] Close Spark session at pipeline finish (#310)

Author: Jeremy Beard

core/src/main/java/com/cloudera/labs/envelope/run/Runner.java

@@ -135,6 +135,7 @@ public void run(Config config) throws Exception {
     finally {
       shutdownThreadPool();
       shutdownSecurity();
+      Contexts.closeSparkSession();
     }
 
     notifyPipelineFinished();