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