[SPARK-23038][TEST] Update docker/spark-test (JDK/OS)

## What changes were proposed in this pull request?

This PR aims to update the followings in `docker/spark-test`.

- JDK7 -> JDK8
Spark 2.2+ supports JDK8 only.

- Ubuntu 12.04.5 LTS(precise) -> Ubuntu 16.04.3 LTS(xeniel)
The end of life of `precise` was April 28, 2017.

## How was this patch tested?

Manual.

* Master
```
$ cd external/docker
$ ./build
$ export SPARK_HOME=...
$ docker run -v $SPARK_HOME:/opt/spark spark-test-master
CONTAINER_IP=172.17.0.3
...
18/01/11 06:50:25 INFO MasterWebUI: Bound MasterWebUI to 172.17.0.3, and started at http://172.17.0.3:8080
18/01/11 06:50:25 INFO Utils: Successfully started service on port 6066.
18/01/11 06:50:25 INFO StandaloneRestServer: Started REST server for submitting applications on port 6066
18/01/11 06:50:25 INFO Master: I have been elected leader! New state: ALIVE
```

* Slave
```
$ docker run -v $SPARK_HOME:/opt/spark spark-test-worker spark://172.17.0.3:7077
CONTAINER_IP=172.17.0.4
...
18/01/11 06:51:54 INFO Worker: Successfully registered with master spark://172.17.0.3:7077
```

After slave starts, master will show
```
18/01/11 06:51:54 INFO Master: Registering worker 172.17.0.4:8888 with 4 cores, 1024.0 MB RAM
```

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #20230 from dongjoon-hyun/SPARK-23038.

external/docker/spark-test/base/Dockerfile

@@ -15,14 +15,14 @@
 # limitations under the License.
 #
 
-FROM ubuntu:precise
+FROM ubuntu:xenial
 
 # Upgrade package index
-# install a few other useful packages plus Open Jdk 7
+# install a few other useful packages plus Open Jdk 8
 # Remove unneeded /var/lib/apt/lists/* after install to reduce the
 # docker image size (by ~30MB)
 RUN apt-get update && \
-    apt-get install -y less openjdk-7-jre-headless net-tools vim-tiny sudo openssh-server && \
+    apt-get install -y less openjdk-8-jre-headless iproute2 vim-tiny sudo openssh-server && \
     rm -rf /var/lib/apt/lists/*
 
 ENV SCALA_VERSION 2.11.8