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