Override entrypoint when shell enabled in Docker.

Review: https://reviews.apache.org/r/25403
Add Docker pull to docker abstraction.

Review: https://reviews.apache.org/r/25523
Added per-container egress rate limit for port mapping network isolator.

When used, the network isolator will install HTB Qdiscs to containers to
control egress traffic throughput to not go more than the specified
value.

Review: https://reviews.apache.org/r/25488
Updated allocator to offer cpu only or memory only resources.

As already explained in JIRA MESOS-1688, there are schedulers
allocating memory only for the executor and not for tasks. For tasks
only CPU resources are allocated in this case. Such a scheduler does
not get offered any idle CPUs if the slave has nearly used up all
memory. This can easily lead to a dead lock (in the application, not
in Mesos).

Simple example:
1. Scheduler allocates all memory of a slave for an executor
2. Scheduler launches a task for this executor (allocating 1 CPU)
3. Task finishes: 1 CPU , 0 MB memory allocatable.
4. No offers are made, as no memory is left. Scheduler will wait for
offers forever. Dead lock in the application.

To fix this problem, offers must be made if CPU resources are
allocatable without considering allocatable memory.

Review: https://reviews.apache.org/r/25035
Added CHANGELOG for 0.20.1.
Check for variadic template support

Review: https://reviews.apache.org/r/25261
Added a removeExecutor helper in the Master.

Review: https://reviews.apache.org/r/25565
Minor cleanups to the Master code.

Review: https://reviews.apache.org/r/25566
MESOS-1392: MasterDetector now returns a None when it cannot read the content of the ZNode it has detected.

Review: https://reviews.apache.org/r/25663
Modify Docker::pull to call inspect after pull.

Review: https://reviews.apache.org/r/25758
Updated CHANGELOG for 0.20.1-rc3.
Replace os::Release with Version.

Review: https://reviews.apache.org/r/25755
Updated callers to os::release in mesos.

Review: https://reviews.apache.org/r/25780
Fixed a signed comparision issue in Version.
Fixed example frameworks to launch tasks based on offered resources.

Review: https://reviews.apache.org/r/25801
Fixed example frameworks to fail fast on an unexpected status udpate.

Review: https://reviews.apache.org/r/25805
Hold on to unacknowledged terminal tasks in the Master.

Review: https://reviews.apache.org/r/25567
Added tests for terminal unacknowledged tasks in the Master.

Review: https://reviews.apache.org/r/25568
Fixed a segfault regression in the master.

Review: https://reviews.apache.org/r/25810
Updated a comment regarding GCC 4.7 in configure.ac and some syntax
formatting.

Review: https://reviews.apache.org/r/25218