Fix docgen with correct path to gem's

Fixes #2251
Add creation of zipfile for the artifacts

Fixes #2221
clear the gem paths before the test rake tasks so they find previously installed plugins

Fixes #2235
Add doc generation tasks to logstash

Fixes #2256
Make the test plugins install throw bundler, so we don't have to install them every time we run the test (it makes the thing slow). Refactor also the bundler install task into a single one, making calls to it when necessary.

Fixes #2237
make sure the bundler task is just used once while bootstraping tests

Fixes #2237
make sure the fail fast test also have the bootstrap for tests done

Fixes #2237