NXCM-5194: Making plugin prefix unique

As otherwise, the 3 runs in IT would produce illegal
3 deploys with same plugin prefix and different
artifact IDs
NXCM-5194: Exposing NexusClient

To allow ITs access to other needed subsystems
NXCM-5194: ITs

Two ITs introduced, one exercising "Deferred deploy" while
other the "staging".

Both are checking for presence and validity of the deployed
G level repository metadata by fetching and parsing it's content
and asserting expected entries.