NEXUS-7586: No DEBUG output and logging cleanup.

Spotted during trying to figure out timeout issue in V3 Staging Client: while code contains DEBUG logs, it is impossible to make them appear in "real" maven build (ie. by using `-X` on CLI). Multiple issues fixed:
* maven log level not propagated to SLF4j (it is not aware it needs to log at DEBUG)
* NX Client debug logging set from SLF4J logging, but see above (basically, was never triggered)
* still, DEBUG lines are not logged, as slf4j-simple does NOT log them :)
* overly messy use of loggers in non-mojo components, somewhere Plexus Logging, somewhere Mojo Log is expected/passed over. Cleaned up and converted to SLF4J Logger, to have it aligned with other (dependency) components, like Nexus Client (that also uses SLF4J for logging).
* updated log use to use `{}` placeholders.

Issue
https://issues.sonatype.org/browse/NEXUS-7586

CI
http://bamboo.s/browse/NX-MVNF9