CLUSTER-39. bento shell command suspected to fail on bash 4.3

 - Normalize bento dev script.
 - Beef-up bento-env and bento scripts.
 - Introduce conf/bento-cluster.version and reduce the scope of Maven assembly filtering to that file.
 - Remove mutate_path(), which did not work with Bash 4.3
 - Also fixes CLUSTER-33 (remove stale PID file if possible).

issue: https://jira.kiji.org/browse/CLUSTER-39
review: https://review.kiji.org/r/1906