[SPARK-21403][MESOS] fix --packages for mesos

## What changes were proposed in this pull request?
Fixes --packages flag for mesos in cluster mode. Probably I will handle standalone and Yarn in another commit, I need to investigate those cases as they are different.

## How was this patch tested?
Tested with a community 1.9 dc/os cluster. packages were successfully resolved in cluster mode within a container.

andrewor14  susanxhuynh ArtRand srowen  pls review.

Author: Stavros Kontopoulos <st.kontopoulos@gmail.com>

Closes #18587 from skonto/fix_packages_mesos_cluster.