WIP NEXUS-6323: Replace plx cipher

Phasing out plexus cipher, using newly introduced component in PR
https://github.com/sonatype/sisu-goodies/pull/27

Issue
https://issues.sonatype.org/browse/NEXUS-6323

CI
http://bamboo.s/browse/NX-OSSF166
This branch needs to be updated wrt. master - the BCPluginEventHandler class was removed last week and should not be re-introduced.