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
Add missing rubysl (rubinius libs) support.

This was already present in 1.x and 1.5 branches; not sure why master
was missing it.
Wrapper for bin/logstash plugin Closes #1969

Fixes #2017
Fixed usage comments

Fixes #2017
Use exec instead of sourcing

Fixes #2017
Fix typo in install

Fixes #2019
Add init scripts to rpm and deb

Fixes #2018