SPARK-3330 [BUILD] Successive test runs with different profiles fail SparkSubmitSuite

Maven-based Jenkins builds have been failing for a while:
https://amplab.cs.berkeley.edu/jenkins/view/Spark/job/Spark-Master-Maven-with-YARN/480/HADOOP_PROFILE=hadoop-2.4,label=centos/console

One common cause is that on the second and subsequent runs of "mvn clean test", at least two assembly JARs will exist in assembly/target. Because assembly is not a submodule of parent, "mvn clean" is not invoked for assembly. The presence of two assembly jars causes spark-submit to fail.