[SPARK-15509][ML][SPARKR] R MLlib algorithms should support input columns "features" and "label"

https://issues.apache.org/jira/browse/SPARK-15509

## What changes were proposed in this pull request?

Currently in SparkR, when you load a LibSVM dataset using the sqlContext and then pass it to an MLlib algorithm, the ML wrappers will fail since they will try to create a "features" column, which conflicts with the existing "features" column from the LibSVM loader. E.g., using the "mnist" dataset from LibSVM:
`training <- loadDF(sqlContext, ".../mnist", "libsvm")`
`model <- naiveBayes(label ~ features, training)`
This fails with:
```
16/05/24 11:52:41 ERROR RBackendHandler: fit on org.apache.spark.ml.r.NaiveBayesWrapper failed
Error in invokeJava(isStatic = TRUE, className, methodName, ...) :
  java.lang.IllegalArgumentException: Output column features already exists.
	at org.apache.spark.ml.feature.VectorAssembler.transformSchema(VectorAssembler.scala:120)
	at org.apache.spark.ml.Pipeline$$anonfun$transformSchema$4.apply(Pipeline.scala:179)
	at org.apache.spark.ml.Pipeline$$anonfun$transformSchema$4.apply(Pipeline.scala:179)
	at scala.collection.IndexedSeqOptimized$class.foldl(IndexedSeqOptimized.scala:57)
	at scala.collection.IndexedSeqOptimized$class.foldLeft(IndexedSeqOptimized.scala:66)
	at scala.collection.mutable.ArrayOps$ofRef.foldLeft(ArrayOps.scala:186)
	at org.apache.spark.ml.Pipeline.transformSchema(Pipeline.scala:179)
	at org.apache.spark.ml.PipelineStage.transformSchema(Pipeline.scala:67)
	at org.apache.spark.ml.Pipeline.fit(Pipeline.scala:131)
	at org.apache.spark.ml.feature.RFormula.fit(RFormula.scala:169)
	at org.apache.spark.ml.r.NaiveBayesWrapper$.fit(NaiveBayesWrapper.scala:62)
	at org.apache.spark.ml.r.NaiveBayesWrapper.fit(NaiveBayesWrapper.sca
The same issue appears for the "label" column once you rename the "features" column.
```
The cause is, when using `loadDF()` to generate dataframes, sometimes it’s with default column name `“label”` and `“features”`, and these two name will conflict with default column names `setDefault(labelCol, "label")` and ` setDefault(featuresCol, "features")` of `SharedParams.scala`

## How was this patch tested?

Test on my local machine.

Author: Xin Ren <iamshrek@126.com>

Closes #13584 from keypointt/SPARK-15509.