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/