Updated the Mesos C++ Style Guide.

Review: https://reviews.apache.org/r/25274
Update deploy_dir from localstate to sysconf

Updates the mesos-env files to install to /etc/mesos vs. /var/lib/deploy/mesos.

Review: https://reviews.apache.org/r/25447
Fixed line comments end punctuation in Mesos source.

Review: https://reviews.apache.org/r/23707
Fixed line comments end punctuation in stout.

Review: https://reviews.apache.org/r/23708
Fixed line comments end punctuation in libprocess.

Review: https://reviews.apache.org/r/23709
Adds Qubit to PoweredByMesos list.
Fix protobuf detection on systems with Python 3 as default (part2)

MESOS-1774

Review: https://reviews.apache.org/r/25439
Send pending tasks during re-registration.

Review: https://reviews.apache.org/r/25371
Made the GarbageCollector injectable into the Slave.

Review: https://reviews.apache.org/r/25372
Added a test for sending pending tasks during re-registration.

Review: https://reviews.apache.org/r/25373
Fix git clean -xdf skipping leveldb, removes internal .git dirs
from leveldb tarball via 'git archive --format=tar.gz
--prefix=leveldb/ origin/master > leveldb.tar.gz'

MESOS-1764

Review: https://reviews.apache.org/r/25508
Fixed command executor path check

Review: https://reviews.apache.org/r/25205
Made sure IPv6 is disabled for port mapping network isolator.

Review: https://reviews.apache.org/r/25512
Fixed authorization tests to properly deal with registration retries.

Review: https://reviews.apache.org/r/25516
Fixed trailing whitespace in libprocess.
Added support for catching trailing white spaces in the style checker.

Review: https://reviews.apache.org/r/25526
Increased session timeouts for ZooKeeper related tests.

- On slower machines sometimes the zookeeper c client times out where we aren't expecting because either the test server or the client is too slow to respond. Increasing this value helps mitigate the problem.

Review: https://reviews.apache.org/r/25487
Pulled the log line in ZooKeeperTestServer::shutdownNetwork() to above the shutdown call.
- When debugging zookeeper related tests it's often more useful to know when the tests is shutting down the ZK server to reason about the order of events. Otherwise client disconnections are often logged before this "shutdown" line and can be confusing.

Review: https://reviews.apache.org/r/25511
First try to fix #1571. Propagate shutdown timeout to Executor and CommandExecutor via env variable. Remove escalation constant for command executor.
Second try to fix 1571.