Allowed co-mounted cgroup subsystems to enable Mesos on machines with
systemd.

Review: https://reviews.apache.org/r/25858
Refactored the environment-based test filtering.

Review: https://reviews.apache.org/r/25569
Removed duplicate error messaging in Docker::create.
Added support for getting socket diagnosis information.

Review: https://reviews.apache.org/r/25948
Added variadic strings join.

There is a second version of the variadic join which takes a reference
to a stringstream as a parameter. This is handy when strings::join is
just a part of a larger string manipulation.

Review: https://reviews.apache.org/r/25789