[SPARK-18686][SPARKR][ML] Several cleanup and improvements for spark.logit.

## What changes were proposed in this pull request?
Several cleanup and improvements for ```spark.logit```:
* ```summary``` should return coefficients matrix, and should output labels for each class if the model is multinomial logistic regression model.
* ```summary``` should not return ```areaUnderROC, roc, pr, ...```, since most of them are DataFrame which are less important for R users. Meanwhile, these metrics ignore instance weights (setting all to 1.0) which will be changed in later Spark version. In case it will introduce breaking changes, we do not expose them currently.
* SparkR test improvement: comparing the training result with native R glmnet.
* Remove argument ```aggregationDepth``` from ```spark.logit```, since it's an expert Param(related with Spark architecture and job execution) that would be used rarely by R users.

## How was this patch tested?
Unit tests.

The ```summary``` output after this change:
multinomial logistic regression:
```
> df <- suppressWarnings(createDataFrame(iris))
> model <- spark.logit(df, Species ~ ., regParam = 0.5)
> summary(model)
$coefficients
             versicolor  virginica   setosa
(Intercept)  1.514031    -2.609108   1.095077
Sepal_Length 0.02511006  0.2649821   -0.2900921
Sepal_Width  -0.5291215  -0.02016446 0.549286
Petal_Length 0.03647411  0.1544119   -0.190886
Petal_Width  0.000236092 0.4195804   -0.4198165
```
binomial logistic regression:
```
> df <- suppressWarnings(createDataFrame(iris))
> training <- df[df$Species %in% c("versicolor", "virginica"), ]
> model <- spark.logit(training, Species ~ ., regParam = 0.5)
> summary(model)
$coefficients
             Estimate
(Intercept)  -6.053815
Sepal_Length 0.2449379
Sepal_Width  0.1648321
Petal_Length 0.4730718
Petal_Width  1.031947
```

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #16117 from yanboliang/spark-18686.

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