NEXUS-5755: Kill PLX from config (and more)

Cleanup of "nx config framework", Plexus removal.

Branched from 
https://github.com/sonatype/nexus-oss/pull/155

Once that above merged, the real diff will be visible here. 

Contains PR
https://github.com/sonatype/nexus-oss/pull/152

Issue
https://issues.sonatype.org/browse/NEXUS-5755

CI
https://bamboo.zion.sonatype.com/browse/NX-OSSF4