Add hack for obtaining cgroup stats

Docker mounts cgroup stats in the wrong place (i.e., inconsistently with
the hierarchy in /proc/1/cgroup). This breaks the logic for traversing
from /proc/self/cgroup to the stats for a process running inside a
container. In core Elasticsearch, an undocumented hack was added to
support this, for now. Thus, for cgroup stats to be available from
Elasticsearch running inside a Docker container, the Elasticsearch
process should be started with `-Des.cgroups.hierarchy.override=/`. This
commit adds support for this hack.

build/elasticsearch/bin/es-docker

@@ -22,4 +22,6 @@ do
     fi
 done < <(env)
 
+echo "-Des.cgroups.hierarchy.override=/" >> config/jvm.options
+
 exec bin/elasticsearch ${es_opts}