[SPARK-18438][SPARKR][ML] spark.mlp should support RFormula.

## What changes were proposed in this pull request?
```spark.mlp``` should support ```RFormula``` like other ML algorithm wrappers.
BTW, I did some cleanup and improvement for ```spark.mlp```.

## How was this patch tested?
Unit tests.

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #15883 from yanboliang/spark-18438.

(cherry picked from commit 95eb06bd)
Signed-off-by: Yanbo Liang <ybliang8@gmail.com>