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
- added a `dripmain.rb` file for a good JVM warmup. see https://github.com/ninjudd/drip/wiki/JRuby and https://github.com/jruby/jruby/blob/master/core/src/main/java/org/jruby/main/DripMain.java

Now vendored JRuby mode is the default, the `USE_RUBY=1` environment var allows to use the local ruby interpreter (whichever Ruby it is inclusing JRuby)

To use Drip, either set the `USE_DRIP=1` environment var or set `JAVACMD=`which drip`.

In my environment, the fastest dev setup for running tests and specs is having JRuby locally installed (I personnaly use rbenv) and launch logstash with `USE_RUBY=1`. To add drip I use both `USE_RUBY=1` and `USE_DRIP=1`.

One caveat with drip I noticed is that unlike nailgun, drip does not reuse the same JVM, once your app quits, drip will launch another JVM. This means that if, for example, the JVM starup+warming takes 5 seconds, it will take another five seconds after your app exited for the drip JVM to be ready. This means that if you try to re-run logstash right after it exited, you might still have a startup delay. There 

I check the drip bin in https://github.com/ninjudd/drip/blob/master/bin/drip and setting the `DRIP_POOL` var to > 1 actually works and spins multiple JVMs, but the initial startup time will take `DRIP_POOL` * the JVM startup time.  `DRIP_POOL` is not externally settable, you have to edit the shell script.