[NEXUS-5892] Add groovy provider plugin
Use buildsupport 4-SNAPSHOT, drop duplicate configuration, change to use latest license plugin GA
Merge pull request #139 from sonatype/NEXUS-5892-groovy-provider-plugin

REVIEW: [NEXUS-5892] Add groovy provider plugin
Add header
Update to buildsupport 4
NEXUS-4997: Upgrade step added

This step will leave SSL only if both TLS and SSL
were checked.

Also, in this case will emit a WARNING in log explaining
what happened.
Merge remote-tracking branch 'origin/master' into remove-god-object