Refactored ZooKeeperImpl into ZooKeeperProcess.

See MESOS-1318 for more details.

Review: https://reviews.apache.org/r/21182
Fixed a bug with containerization in MesosTest.

Review: https://reviews.apache.org/r/21180
Added a test to ensure a recovered slave that does not re-register is removed.

Review: https://reviews.apache.org/r/21176
Added a test that ensure a recovered slave is not removed.

Review: https://reviews.apache.org/r/21186
Added a test to ensure a non-strict registry is write-only.

Review: https://reviews.apache.org/r/21189
Added user field to CommandInfo.

Enables individual executors and tasks to run as a specific user. If
the user field is present both in FrameworkInfo and here, the
CommandInfo user value takes precedence.

Review: https://reviews.apache.org/r/21167
Introduced a HealthCheck protobuf.

Review: https://reviews.apache.org/r/21052
Added an optional timeout to metrics collection.

Review: https://reviews.apache.org/r/21092
Updated post-reviews to catch exceptions and return None if ignoring
errors in execute.

Review: https://reviews.apache.org/r/21086