logstash boot sequence cleanup & drip support fix

- big cleanup of boot shell scripts
- fixed drip support, works with both vendored JRuby and using local JRuby
- deprecated USE_JRUBY environment var
- added USE_RUBY and USE_DRIP environment vars
- centralized gem path handling in LogStash::Environment

Now vendored JRuby mode is the default, the `USE_RUBY=1` environment var allows to use the local ruby interpreter. 

To use Drip, either set the `USE_DRIP=1` environment var or set `JAVACMD=`which drip`.