Improvements to plugin manager

- Use better installer method to avoid documentation installation
  With the previous method it would install the documentation which didn't exist.

- Create Fake gemspec info for logstash to help dependency management for plugins
  Previously we would create a real gem and install it to aid in dependency management for plugins and logstash
  Now we create a fake gemspec on the fly with the real version of logstash avoiding having to build it every time.
Move logstash gemspec loading into environment module

Previously the loading of the logstash gemspec was limited to the plugin manager
After more tests we need the gemspec to be loaded for any action with the plugins
Moving it to the environment module makes it available for more generic loading

Make sure plugin manager uses new function for loading gemspec
this should fix logstash #1738
Closes #1738
test and fix for LOGSTASH-2288