Add /var/log/logstash as an empty directory.

Testing deb package

    % dpkg -c build/logstash_2.0.0.dev-1_all.deb | grep var/log/logstash
    drwxrwxr-x 0/0               0 2014-12-01 22:29 ./var/log/logstash/

Testing rpm package

    % rpm -qvlp build/logstash-2.0.0.dev-1.noarch.rpm| grep var/log/logstash
    drwxrwxr-x    2 root    root                        0 Dec  1 22:34 /var/log/logstash

Fixes #2146