Make withPlugins work for installing any plugins

[hakanai]

There are 3 plugins I would have liked to install into Elasticsearch before it starts up, but withPlugins appears to be hard-coded to install one plugin by name.

Ideally it would be nice to be able to pass file paths, so that I can install a plugin that isn't in the plugin repository, because one of the things we're testing is that plugin itself, and we don't plan to include it in the plugin repository.

[amirkibbar]

unfortunately this proved to be too much work to both adapt the plugin to ES 5.x and continue to support plugin installation, so I removed the withPlugins option altogether. If you'd like to contribute code that installs any plugin I'd be more than happy to add it