External path option for the logstash installation + helper scripts

Added an optional parameter that let you select the logstash installation path, or just use the Dir.pwd as it was before.

Useful to run this test regularly for different checkouts.

It also adds some scripting useful to run this in regular basis to different clones.
I don't think we should pass the logstash home path as a cmd line argument, IMO it should be an ENV var.