Re-enable encrypted keys for Logstash beats inputs

In #46 we deactivated encryption for TLS keys for Beats inputs in Logstash. We ran through a lot of tests for different combinations of input plugin, Logstash and JRE but we couldn't find a variant that reliably works.

Maybe in the future this will change so I'm opening this issue so we don't forget about it. (And to have a link to the removed code)