[SPARK-18335][SPARKR] createDataFrame to support numPartitions parameter

## What changes were proposed in this pull request?

To allow specifying number of partitions when the DataFrame is created

## How was this patch tested?

manual, unit tests

Author: Felix Cheung <felixcheung_m@hotmail.com>

Closes #16512 from felixcheung/rnumpart.

(cherry picked from commit b0e8eb6d)
Signed-off-by: Shivaram Venkataraman <shivaram@cs.berkeley.edu>