Revert "[SPARK-15271][MESOS] Allow force pulling executor docker images"

...spark/scheduler/cluster/mesos/MesosClusterScheduler.scala

...er/cluster/mesos/MesosCoarseGrainedSchedulerBackend.scala

...uler/cluster/mesos/MesosFineGrainedSchedulerBackend.scala

...k/scheduler/cluster/mesos/MesosSchedulerBackendUtil.scala

...uster/mesos/MesosCoarseGrainedSchedulerBackendSuite.scala

...cluster/mesos/MesosFineGrainedSchedulerBackendSuite.scala

dev/deps/spark-deps-hadoop-2.2

dev/deps/spark-deps-hadoop-2.3

dev/deps/spark-deps-hadoop-2.4

dev/deps/spark-deps-hadoop-2.6

dev/deps/spark-deps-hadoop-2.7

docs/_config.yml

docs/running-on-mesos.md

pom.xml