Added instructions for getting started with CentOS.

Review: https://reviews.apache.org/r/25272
Added Resources support to return the port range for the given number of ports.

Review: https://reviews.apache.org/r/24810
Updated the container failed to launch error message in the slave.

Review: https://reviews.apache.org/r/25273
Count pending tasks as staging in the slave.

Review: https://reviews.apache.org/r/25302
Fixed a bug in the staging tasks metric.

Review: https://reviews.apache.org/r/25303
Removed an unnecessarily introduced Option.

Review: https://reviews.apache.org/r/25304
Flattened the executor resoures when launching executors.

Review: https://reviews.apache.org/r/25306
Fixed the memory limitation construction.
Improved the error message when destroying a container.

Review: https://reviews.apache.org/r/25344
Minor update to include package config file

Minor update to include package config file

Review: https://reviews.apache.org/r/25355
Added optional --offer_timeout to rescind unused offers.

The ability to set an offer timeout helps prevent unfair resource
allocations in the face of frameworks that hoard offers, or that
accidentally drop offers.

When optimistic offers are added, hoarding will not affect the
fairness for other frameworks.

Review: https://reviews.apache.org/r/22066
Override entrypoint when shell enabled in Docker