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.
NEXUS-6757: Workaround for Verifier bug

Adding same "-D" option with opposite value to
override it, instead clearing CLI options as it
would make mvn use user's settings.xml instead of
the one provided by Verifier/test
Merge pull request #75 from sonatype/NEXUS-6757-two-shots

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