[SPARK-21694][MESOS] Support Mesos CNI network labels

JIRA ticket: https://issues.apache.org/jira/browse/SPARK-21694

## What changes were proposed in this pull request?

Spark already supports launching containers attached to a given CNI network by specifying it via the config `spark.mesos.network.name`.

This PR adds support to pass in network labels to CNI plugins via a new config option `spark.mesos.network.labels`. These network labels are key-value pairs that are set in the `NetworkInfo` of both the driver and executor tasks. More details in the related Mesos documentation:  http://mesos.apache.org/documentation/latest/cni/#mesos-meta-data-to-cni-plugins

## How was this patch tested?

Unit tests, for both driver and executor tasks.
Manual integration test to submit a job with the `spark.mesos.network.labels` option, hit the mesos/state.json endpoint, and check that the labels are set in the driver and executor tasks.

ArtRand skonto

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

Closes #18910 from susanxhuynh/sh-mesos-cni-labels.