[SPARK-18810][SPARKR] SparkR install.spark does not work for RCs, snapshots

## What changes were proposed in this pull request?

Support overriding the download url (include version directory) in an environment variable, `SPARKR_RELEASE_DOWNLOAD_URL`

## How was this patch tested?

unit test, manually testing
- snapshot build url
  - download when spark jar not cached
  - when spark jar is cached
- RC build url
  - download when spark jar not cached
  - when spark jar is cached
- multiple cached spark versions
- starting with sparkR shell

To use this,
```
SPARKR_RELEASE_DOWNLOAD_URL=http://this_is_the_url_to_spark_release_tgz

 R
```
then in R,
```
library(SparkR) # or specify lib.loc
sparkR.session()
```

Author: Felix Cheung <felixcheung_m@hotmail.com>

Closes #16248 from felixcheung/rinstallurl.

(cherry picked from commit 8a51cfdc)
Signed-off-by: Shivaram Venkataraman <shivaram@cs.berkeley.edu>