[SPARK-20078][MESOS] Mesos executor configurability for task name and labels

## What changes were proposed in this pull request?

Adding configurable mesos executor names and labels using `spark.mesos.task.name` and `spark.mesos.task.labels`.

Labels were defined as `k1:v1,k2:v2`.

mgummelt

## How was this patch tested?

Added unit tests to verify labels were added correctly, with incorrect labels being ignored and added a test to test the name of the executor.

Tested with: `./build/sbt -Pmesos mesos/test`

Please review http://spark.apache.org/contributing.html before opening a pull request.

Author: Kalvin Chau <kalvin.chau@viasat.com>

Closes #17404 from kalvinnchau/mesos-config.