[SPARK-11728] Replace example code in ml-ensembles.md using include_example

[yinxusen]

JIRA issue https://issues.apache.org/jira/browse/SPARK-11728.

The ml-ensembles.md file contains OneVsRestExample. Instead of writing new code files of two OneVsRestExamples, I use two existing files in the examples directory, they are OneVsRestExample.scala and JavaOneVsRestExample.scala.

[SparkQA]

Test build #45929 has finished for PR 9716 at commit 9ba1d4a.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):\n * public class JavaGradientBoostedTreeClassifierExample\n * public class JavaGradientBoostedTreeRegressorExample\n * public class JavaRandomForestClassifierExample\n * public class JavaRandomForestRegressorExample\n

[mengxr]

This is different from current example code in user guide, where we use sqlContext.read.format("libsvm").load("data/mllib/sample_libsvm_data.txt"). Please find other places and switch to using "libsvm" data source instead.

[mengxr]

LGTM except one issue commented inline. Please keep the example code the same while moving them around. It is easier for review.

[yinxusen]

Fixed, sorry for the carelessness.

[SparkQA]

Test build #46164 has finished for PR 9716 at commit 4d9cffb.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):\n * public class JavaGradientBoostedTreeClassifierExample\n * public class JavaGradientBoostedTreeRegressorExample\n * public class JavaRandomForestClassifierExample\n * public class JavaRandomForestRegressorExample\n

[SparkQA]

Test build #46162 has finished for PR 9716 at commit 2ed613a.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):\n * public class JavaGradientBoostedTreeClassifierExample\n * public class JavaGradientBoostedTreeRegressorExample\n * public class JavaRandomForestClassifierExample\n * public class JavaRandomForestRegressorExample\n

[mengxr]

Merged into master and branch-1.6. Thanks!