## What changes were proposed in this pull request?
Add an install_spark function to the SparkR package. User can run `install_spark()` to install Spark to a local directory within R.
Updates:
Several changes have been made:
- `install.spark()`
- check existence of tar file in the cache folder, and download only if not found
- trial priority of mirror_url look-up: user-provided -> preferred mirror site from apache website -> hardcoded backup option
- use 2.0.0
- `sparkR.session()`
- can install spark when not found in `SPARK_HOME`
## How was this patch tested?
Manual tests, running the check-cran.sh script added in #14173.
Author: Junyang Qian <junyangq@databricks.com>
Closes#14258 from junyangq/SPARK-16579.
214ba66a03