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
Fixed flaky MasterTest.LaunchDuplicateOfferTest.

A couple of races could occur in the "launch tasks on multiple offers"
tests where recovered resources from purposely-failed invocations turned
into a subsequent resource offer and oversaturated the expect's.

Review: https://reviews.apache.org/r/25588
Fixed the flaky FaultToleranceTest.ReconcilePendingTasks.

Review: https://reviews.apache.org/r/25604
Added a Version class to stout.

Currently there is no facility in Mesos for checking compatibility of
various Mesos components that could have been built at different
times with potentially different Mesos versions.  This requirement is
especially important for doing various compatibility checks between
Mesos and Mesos modules (WIP).

Review: https://reviews.apache.org/r/25597
Enabled bridge network for Docker Containerizer.

Review: https://reviews.apache.org/r/25270