This fixes the `SparkSubmitSuite` failure by setting `<spark.ui.port>0</spark.ui.port>` in the Maven build, to match the SBT build. This avoids a port conflict which causes failures.

(This also updates the `scalatest` plugin off of a release candidate, to the identical final release.)

Author: Sean Owen <sowen@cloudera.com>

Closes #2328 from srowen/SPARK-3404 and squashes the following commits:

512d782 [Sean Owen] Set spark.ui.port=0 in Maven scalatest config to match SBT build and avoid SparkSubmitSuite failure due to port conflict