NEXUS-6757: Fix for two-shots workflow and IT

As the deploy-staged mojo was untested so far, and it got broken
during refactoring happened for 1.6. The IT now covers both
"style" of projects (set up in maven2-way and maven3-way) and
executes them with m2 and m3.

Issue
https://issues.sonatype.org/browse/NEXUS-6757

CI
TBD