## What changes were proposed in this pull request?

This PR aims to add `setLogLevel` function to SparkR shell.

**Spark Shell**
```scala
scala> sc.setLogLevel("ERROR")
```

**PySpark**
```python
>>> sc.setLogLevel("ERROR")
```

**SparkR (this PR)**
```r
> setLogLevel(sc, "ERROR")
NULL
```

## How was this patch tested?

Pass the Jenkins tests including a new R testcase.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #12547 from dongjoon-hyun/SPARK-14780.