Some documentation generation fixes

- Add docgen command to use internal libraries
- Fix loading of base files due to plugin path changes
- Fix loading of jar dependency files
- Remove kramdown gem and actions
- Cleanup the generated synopsis a bit

Fixes #2009
added rake vendor:force:gems option for forcing a bundle install

Fixes #1952
Use returns instead of exit

Need to return the exit code and let the runner app perform the exit

Fixes #1838
Only print 'relaunching' if run under debug.

Fixes #1995
Fix tarball packaging to be rooted in logstash-<VERSION>

Fixes #1982

Fixes #1995
Remove some vendor tasks from vendor:all

kafka, geoip, elasticsearch, and collectd moved into separate plugins

Fixes #1995