[SPARK-21149][R] Add job description API for R

## What changes were proposed in this pull request?

Extend `setJobDescription` to SparkR API.

## How was this patch tested?

It looks difficult to add a test. Manually tested as below:

```r
df <- createDataFrame(iris)
count(df)
setJobDescription("This is an example job.")
count(df)
```

prints ...

![2017-06-22 12 05 49](https://user-images.githubusercontent.com/6477701/27415670-2a649936-5743-11e7-8e95-312f1cd103af.png)

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #18382 from HyukjinKwon/SPARK-21149.

R/pkg/NAMESPACE

@@ -75,7 +75,8 @@ exportMethods("glm",
 # Job group lifecycle management methods
 export("setJobGroup",
        "clearJobGroup",
-       "cancelJobGroup")
+       "cancelJobGroup",
+       "setJobDescription")
 
 # Export Utility methods
 export("setLogLevel")

R/pkg/R/sparkR.R

@@ -535,6 +535,23 @@ cancelJobGroup <- function(sc, groupId) {
   }
 }
 
+#' Set a human readable description of the current job.
+#'
+#' Set a description that is shown as a job description in UI.
+#'
+#' @param value The job description of the current job.
+#' @rdname setJobDescription
+#' @name setJobDescription
+#' @examples
+#'\dontrun{
+#' setJobDescription("This is an example job.")
+#'}
+#' @note setJobDescription since 2.3.0
+setJobDescription <- function(value) {
+  sc <- getSparkContext()
+  invisible(callJMethod(sc, "setJobDescription", value))
+}
+
 sparkConfToSubmitOps <- new.env()
 sparkConfToSubmitOps[["spark.driver.memory"]]           <- "--driver-memory"
 sparkConfToSubmitOps[["spark.driver.extraClassPath"]]   <- "--driver-class-path"

R/pkg/tests/fulltests/test_context.R

@@ -100,6 +100,7 @@ test_that("job group functions can be called", {
   setJobGroup("groupId", "job description", TRUE)
   cancelJobGroup("groupId")
   clearJobGroup()
+  setJobDescription("job description")
 
   suppressWarnings(setJobGroup(sc, "groupId", "job description", TRUE))
   suppressWarnings(cancelJobGroup(sc, "groupId"))