Purpose:
* manage releases through a minimum number of rake tasks
* simplify building of snapshot builds
* create staged artifacts, candidates for releases, that required no changes to become releases
* this means the snapshot release process will not involve publishing gems, therefore:
* the gem artifacts should only be published to rubygems as a final artifact, at the time of GA
Changes:
* release artifacts no longer depend on gems of core components
* all core components are used locally AS-IS to minimize code changes between snapshot, RC and GA
* `versions.yml` describes the versions of all logstash parts and package
* `rake version:set[version]` manage the yaml file and push the changes to the gemspecs/version.rb files
* `rake version:set_plugin_api[version]` manage the yaml file and push the changes to the gemspecs/version.rb files
* `rake artifact:all` generates SNAPSHOT artifacts: tar.gz, zip, rpm, deb
* `RELEASE=1 rake artifact:all` creates release candidate artifacts + 4 gems: logstash-core, logstash-core-event, logstash-core-event-java and logstash-core-plugin-api
implements #5416 and #5414Fixes#5460