Use nexus-staging-maven-plugin

The nexus-staging-maven-plugin allows you to deploy to
sonatype, and automatically close and release a repository
without having to log into the sonatype UI. This will allow
us to use sonatype in a CD pipeline.
Use BUSYBOX attribute in SystemTestBase

instead of string "busybox".
Fix and add toJsonString() to CLI responses

Fix misnamed classes in toString().
Add toJsonString() to all CLI responses.
Fix mismatched name, repo errors in JobValidator
WIP Respect `--json` switch in CLI tool