Fixed the Balloon Framework Test.

Stop exporting variables that begin with MESOS_ into the mesos
environment. The mesos-master and mesos-slave binaries will not start
if there are unrecognized variables that begin with MESOS_ in the
environment.

Set the LD_LIBRARY_PATH so that the balloon binaries built with
libtool will be able to find all of their pieces and execute properly
without needing to be installed.

Cleanup the cleanup code so we don't unmount a cgroup hierarchy unless
we have fully cleaned it up. This is necessary to prevent a problem
where the hierarchy can never be mounted again.

Update the command line options to mesos-master to pass the hierarchy
and the mesos root directory in that hierarchy.

Clearly report when the balloon hierarchy returns with an unexpected
return code in order to indicate whether the test failed because of a
problem in the baloon framework or within mesos.

From: Eric Biederman <ebiederm@xmission.com>
Review: https://reviews.apache.org/r/13034
Added a test for slave recovery when a scheduler failover happens.

From: Jie Yu <yujie.jay@gmail.com>
Review: https://reviews.apache.org/r/13677
Added slave recovery network partition test.

From: Jie Yu <yujie.jay@gmail.com>
Review: https://reviews.apache.org/r/13723
Fixed slave recovery tests to use 'strict' option by default.

Review: https://reviews.apache.org/r/13676
Fixed slave to not log a warning when it generates TASK_LOST update.

Review: https://reviews.apache.org/r/13715
Fixed SlaveRecovery.Partitioned slave test.

Review: https://reviews.apache.org/r/13729
Removed internal logging from state::checkpoint().

Review: https://reviews.apache.org/r/13698
Added a re-registration time for slaves.

Review: https://reviews.apache.org/r/13743
Fixed a bug that caused Master::~Master() to send messages to the
schedulers and slaves.

- In production, Masters exit by either LOG(FATAL) or exit().
- Also fixed tests that incorrectly relied on the messages sent by
  Master::~Master().

From: Jiang Yan Xu <yan@jxu.me>
Review: https://reviews.apache.org/r/13746
Updated version to 0.15.0.