Re-enable encrypted TLS keys for Logstash Beats input

In #46 we had to disable encryption for TLS keys for the Beats input of Logstash. We went through a lot of different combinations of Logstash, Beats plugin and JRE but we couldn't find a reliabe combination that will work.

So I'm opening this issue so that we don't forget to re-enable if the future brings reliable combinations. (And to have a link to the removed code in case we can re-use it)