Move boilerplate rakelib/* from plugins into a separate library.

All plugins should now have

* a single-line `Rakefile`

        require "logstash/devutils/rake"

* Ensure 'logstash-devutils' is a development dependency

        Gem::Specification.new do |spec|
          ...
          spec.add_development_dependency 'logstash-devutils'
          ...
        end

This will allow 'bundle install' to install everything necessary
to make 'rake' and 'rspec' work from a plugin developer's workspace.