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.
It is already an ENV var, however I added the option to use a parameter too, in this case if someone use the parameter it takes precedence over the ENV var, but one can just use the ENV var and forget.