Cleanup plugins to move to plugin system

Plugins have moved to https://github.com/logstash-plugins/... into
individual repositories. One plugin per repo.

* Add plugin install task and add required plugins for testing
* Adding a plugin task to install plugins
* Added required plugins to a prep task for the test part.

Fixes #1984

Conflicts:
	lib/logstash/filters/multiline.rb
	lib/logstash/outputs/hipchat.rb
	logstash.gemspec