RPM/Deb packages should install a symlink into /usr/local/bin

For ease-of-use, I think this would be more compliant to the "least-surprise-principle" that after installing logstash through official packages, the logstash command is immediately usable.

Doing so could be seemingly easily done by just creating a symlink into /usr/local/bin/logstash pointing to /opt/logstash/bin/logstash so that typical linux installations having /usr/local/bin in their PATH can use logstash immediately.

Thanks a lot!