[SPARK-21778][SQL] Simpler Dataset.sample API in Scala / Java

## What changes were proposed in this pull request?
Dataset.sample requires a boolean flag withReplacement as the first argument. However, most of the time users simply want to sample some records without replacement. This ticket introduces a new sample function that simply takes in the fraction and seed.

## How was this patch tested?
Tested manually. Not sure yet if we should add a test case for just this wrapper ...

Author: Reynold Xin <rxin@databricks.com>

Closes #18988 from rxin/SPARK-21778.