Made GC_DISK_HEADROOM configurable.

Added --gc_disk_headroom to mesos-slave which let's users configure
the disk headroom value used to calculate maximum executor age.

Review: https://reviews.apache.org/r/28681
Add slave id to docker container name prefix.
Add option to disable docker containerizer killing orphans
Add executor for docker containerizer
Integrate docker executor into containerizer.
Re-enable docker recover test.
Add docker_mesos_image flag to slave flags.
Add docker_sock to slave flags
Add option to launch docker containers with helper containers.
Fix docker container naming and tests.
Recover docker containers that launched in containers.