MESOS_NATIVE_JAVA_LIBRARY not set on MesosContainerize tasks with --executor_environmnent_variables

When using --executor_environment_variables, and having MESOS_NATIVE_JAVA_LIBRARY in the environment of mesos-slave, the mesos containerizer does not set MESOS_NATIVE_JAVA_LIBRARY itself.

Relevant code: https://github.com/apache/mesos/blob/14f7967ef307f3d98e3a4b93d92d6b3a56399b20/src/slave/containerizer/containerizer.cpp#L281

It sees that the variable is in the mesos-slave's environment (os::getenv), rather than checking if it is set in the environment variable set.