NXCM-5297: Promotion should print created group ID and other

While the resource that starts promotion does not
emits the (not even created yet) group ID, client
has that info received during "transition monitoring".

Using the newly introduced TransitionListener to grab
that information to present to the user.

Related issue:
https://issues.sonatype.org/browse/NXCM-5297

Moreover, this pull contains complete update of the nexus-maven-plugins,
updating it to Nexus 2.6 and other improvement, such as:
* changed GAs of shuffled things
* removed p-u uses, replaced with Guava
* IT deps fixed (things like pluginkit or testlm)
* removed commented out stuff from POMs, removed unneded cruft
* source imports organized, removed unused cruft