kiji-bento: BENTO-98. Adds option to kiji-env.sh to override existing HADOOP_HOME/HBASE_HOME.

issue: https://jira.kiji.org/browse/BENTO-98
review: https://review.kiji.org/r/2097
commit: https://github.com/kijiproject/kiji-bento/commit/dfc3f70f6f8d1d215b884b8e44f7b879c7110542