## What changes were proposed in this pull request?

Allow user to set sparkr shell command through --conf spark.r.shell.command

## How was this patch tested?

Unit test is added and also verify it manually through
```
bin/sparkr --master yarn-client --conf spark.r.shell.command=/usr/local/bin/R
```

Author: Jeff Zhang <zjffdu@apache.org>

Closes #14744 from zjffdu/SPARK-17178.