Install test plugin dependencies throw a Bundler

Every time we run the test, there is a test task named test:prep that install the plugins throw the logstash command. This makes the test slow, because every new run it installs again the plugins. 

This PR does this installation throw Bundler, like the others dependencies, so each time we run the test, we just pick the previous installation if is the case. It also include an small refactoring, having now only one task to run the bundler install command, making things mode simple.