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
@mrprescott yup, this is wrong: originally I made the change and the @jdillon pointed out that @mcculls is about to remove BC plugin as is and solve it with OSGi. That will be merged in here.