NXCM-5297: Promotion should print created group ID

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.
NXCM-5297: Simplified to new changes.

Removed all the fluff, and using the returned string
to present to user.

If connected to older Nexus, console will show you
"Promoted to group n/a", where new instances
will tell you the actual group ID that was created.
NXCM-5297: Using 2.6 of Nexus Staging

As it holds the needed API change to emit the
group ID
Adapted plugins to Nx 2.6

* 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

And by the way, this fixes NXCM-5297 too, as it was
the whole point: bump nexus staging client to 2.6
NXCM-5297: Improved console message.

Also, applied to RC goal too. Special
message explains why ID is not available.
NXCM-5297: IT

Added IT that verifies the positive outcome (ID received
from modern nexus) and is printed out on console.
Merge pull request #41 from sonatype/nxcm-5297-promotion-group-id

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