Updated --isolation help message for slave.

Now also mentions the --modules flag.

Review: https://reviews.apache.org/r/27560
First version of docs/modules.md.

With bits copied from
https://cwiki.apache.org/confluence/display/MESOS/Mesos+Modules+Developer+Guide.

Review: https://reviews.apache.org/r/27453
Update release guide to mention configure.ac.
Adds GoCardless to Mesos adopters list.
Pass user into Isolator::prepare.

Review: https://reviews.apache.org/r/27557
Permit unprivileged executors to created child cgroups.

MesosContainerizer Isolators using cgroups will chown the cgroup to
the executor user, if specified, enabling the executor to create child
cgroups. Cgroup control files are not chowned so the executor cannot
modify its own cgroups, only control files for child cgroups.

Review: https://reviews.apache.org/r/27558
Fix OsTest.killtreeNoRoot check for reparenting.

Reparenting does not always assign to pid 1 (/sbin/init). If there is
a user init such as init --user with some other pid, this will be the
new parent.

Modify os_tests to check that the subtree has been reparented to a
process different from its original parent (a.k.a. child) and that it
is not a zombie.

Review: https://reviews.apache.org/r/27461
Documentation for filesystem/shared isolator.

Review: https://reviews.apache.org/r/27584
Documentation for namespaces/pid isolator.

Review: https://reviews.apache.org/r/27585
Updated release guide to mention configuration.md.

To update the flags for master, slave and configure.

Review: https://reviews.apache.org/r/27573
Add source and reason to TaskStatus.

Review: https://reviews.apache.org/r/26382/
Add MESOS-343 and MESOS-1143 to CHANGELOG.

Review: https://reviews.apache.org/r/27595/
Added External Containerizer documentation.

Adds a markdown document describing the ExternalContainerizer.

Review: https://reviews.apache.org/r/22169/
MESOS-2038: removed dead code from _runTask().

Review: https://reviews.apache.org/r/27567
CHANGELOG and docs/upgrades.md for 0.21.0.

Review: https://reviews.apache.org/r/27633
Update configure.ac version to 0.22.0
Fetcher uses Hadoop to fetch URIs with unknown schemes.

Review: https://reviews.apache.org/r/27483
Updated docs/configuration.md.

- Copied the output of --help messages from configure, master, and
  slave.
- The option tables are now in html format.
- Removed the explanation of the default configure flags.

Review: https://reviews.apache.org/r/27556
Only push the tag when tagging a release.

Git prefers branches to tags so be explicit.
Updated scheduler driver to exponentially backoff during registration
retries.

Review: https://reviews.apache.org/r/27315