REVIEW [NX-391] integrate pax-logging-logback

Enables Karaf support for logback configuration as well as request logging. Events are written to the usual Nexus log and are also visible/searchable using Karaf's log command. Logging levels can also be set using the Nexus UI or Karaf's log command.

This PR requires pax-logging 1.7.3-SNAPSHOT - all changes needed by Nexus have been committed and we can cut a release when necessary (in the meantime we could always add the OPS4J snapshot repository on OSSRH to the RSO public group and consume the snapshot from there). Note that the assembly replace task (to swap Karaf's default Log4J backend configuration with pax-logging-logback) needs cleaning up so it doesn't have hard-coded versions.

https://issues.sonatype.org/browse/NX-391