install default plugins via bundler

you know, for speed.

This is an attempt to speed `rake artifact` tasks. One of the major factors slowing the process is the Gem::DependencyInstaller#install method, called for each plugin. This method will check the dependencies for the gem, meaning dependencies are checked ~100 times during the rake task.

An alternative is to hand out all the gems to bundler and have it figure out dependencies and install order in 1 go.

The approach also removes the ~100 "system" calls to "bin/plugin install" (each spawns a jvm instance).

This reduces the run time by ~60% in my tests (before: 36 minutes. now: 12 minutes).

I'm not sure if there's any negative impact on this approach, please review.

I tried to use `bundler/cli` to run bundler like it's done in other parts of logstash:

```ruby 
task "install-defaults" do
    require 'bundler/cli'
    ENV['GEM_PATH'] = LogStash::Environment.gem_home
    ENV['GEM_HOME'] = LogStash::Environment.plugins_home
    Bundler::CLI.start(["install", "--clean", "--jobs", "3", "--retry", "10"])
  end
```

However, the current `rake plugin:install-defaults` behaviour is to install the plugins' gems into "vendor/plugin" while the other gems live in "vendor/bundle", and I can't get Bundler to do this. Even setting them separately like the snippet above makes Bundler install all gems to $GEM_PATH, while running Bundler with "--path vendor/bundle" will install _everything_ in that path, not reusing those in "vendor/bundle".