HADOOP_HOME no longer works with fetcher

As of commit "699e638ef80739203ed38ba7238bbc176b107355 Consolidate all fetcher env vars into one that holds a JSON object." the HADOOP_HOME environment variable is no longer set when starting the fetcher, which means that hdfs.hpp's HDFS() will no longer look for the hadoop binary inside the user-provided HADOOP_HOME, whether it was specified via --hadoop_home or the HADOOP_HOME env variable.
This is a regression in functionality from Mesos 0.21 and before, and breaks the use of HDFS when not installed in the slave's PATH. The future fetcher patch(es) for 0.23+ fixes this, but we need something in 0.22 to prevent regression.