[SPARK-17964][SPARKR] Enable SparkR with Mesos client mode and cluster mode

## What changes were proposed in this pull request?

Enabled SparkR with Mesos client mode and cluster mode. Just a few changes were required to get this working on Mesos: (1) removed the SparkR on Mesos error checks and (2) do not require "--class" to be specified for R apps. The logic to check spark.mesos.executor.home was already in there.

sun-rui

## How was this patch tested?

1. SparkSubmitSuite
2. On local mesos cluster (on laptop): ran SparkR shell, spark-submit client mode, and spark-submit cluster mode, with the "examples/src/main/R/dataframe.R" example application.
3. On multi-node mesos cluster: ran SparkR shell, spark-submit client mode, and spark-submit cluster mode, with the "examples/src/main/R/dataframe.R" example application. I tested with the following --conf values set: spark.mesos.executor.docker.image and spark.mesos.executor.home

This contribution is my original work and I license the work to the project under the project's open source license.

Author: Susan X. Huynh <xhuynh@mesosphere.com>

Closes #15700 from susanxhuynh/susan-r-branch.