Include an indication of the CleanUp tasks log file in the nexus.log file when CleanUp task is run

When the Cleanup scheduled task runs, we have almost all the information in the distinct task log. On a clean run it just shows that the task started and ended. Compare this to the clean snapshot task which shows a line where the task log is:

2019-06-12 18:31:17,851-0400 INFO  [quartz-3-thread-6] *SYSTEM org.sonatype.nexus.repository.maven.tasks.RemoveSnapshotsTask - Task log: /Users/jtom/Documents/Work/nexus/OlderNX3/nexus-3.16.1-02/sonatype-work/nexus3/log/tasks/repository.maven.remove-snapshots-20190612183117849.log

When the CleanUp task is run, write a log message to the nexus.log file showing the location of the CleanUp tasks log.

I think this is a worthy addition to cleanup so 1) folks know that more information exists and 2) where.