Containerized TaskManager killed because of lack of memory overhead

Currently, the `-XX:MaxDirectMemorySize` parameter is set as:
`MaxDirectMemorySize = containerMemoryMB - heapSizeMB`
(see https://github.com/apache/flink/blob/7fec4392b21b07c69ba15ea554731886f181609e/flink-runtime/src/main/java/org/apache/flink/runtime/clusterframework/ContaineredTaskManagerParameters.java#L162)

However as explained at
 https://docs.oracle.com/javase/8/docs/technotes/tools/unix/java.html,
`MaxDirectMemorySize` only sets the maximum amount of memory that can be
used for direct buffers, thus the amount of off-heap memory used can be
greater than that value, leading to the container being killed by Mesos
or Yarn as it exceeds the allocated memory.

In addition, users might want to allocate off-heap memory through native
code, in which case they will want to keep some of the container memory
free and unallocated by Flink.

To solve this issue, we currently set the following parameter:

-Dcontainerized.taskmanager.env.FLINK_ENV_JAVA_OPTS='-XX:MaxDirectMemorySize=600m'

which overrides the value that Flink picks (744M in this case) with a lower one to keep some overhead memory in the TaskManager containers. However this is an "ugly" hack as it goes around the clever memory allocation that Flink performs and allows to bypass the sanity checks done in `ContaineredTaskManagerParameters`.