## What changes were proposed in this pull request?
Add `conf` method to get Runtime Config from SparkSession
## How was this patch tested?
unit tests, manual tests
This is how it works in sparkR shell:
```
SparkSession available as 'spark'.
> conf()
$hive.metastore.warehouse.dir
[1] "file:/opt/spark-2.0.0-bin-hadoop2.6/R/spark-warehouse"
$spark.app.id
[1] "local-1466749575523"
$spark.app.name
[1] "SparkR"
$spark.driver.host
[1] "10.0.2.1"
$spark.driver.port
[1] "45629"
$spark.executorEnv.LD_LIBRARY_PATH
[1] "$LD_LIBRARY_PATH:/usr/lib/R/lib:/usr/lib/x86_64-linux-gnu:/usr/lib/jvm/default-java/jre/lib/amd64/server"
$spark.executor.id
[1] "driver"
$spark.home
[1] "/opt/spark-2.0.0-bin-hadoop2.6"
$spark.master
[1] "local[*]"
$spark.sql.catalogImplementation
[1] "hive"
$spark.submit.deployMode
[1] "client"
> conf("spark.master")
$spark.master
[1] "local[*]"
```
Author: Felix Cheung <felixcheung_m@hotmail.com>
Closes#13885 from felixcheung/rconf.
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