CTA front-end now logs duplicate tape copy numbers

If the "cta af ls" command is executed and the listing
encounters duplicate tape copy numbers then the listing
continues with "workaround" number displayed in place
of the duplicates and inow with this commit a log
message is alseo written to

    /var/log/cta/cta-frontent.log

An example log:

2017-07-12T18:04:40.691236+02:00 itdssbuild01 cta-frontend: LVL="Warn" PID="9423" TID="9436" MSG="Found a duplicate tape copy number when listing archive files" archiveFileID="5" duplicateCopyNb="1" workaroundCopyNb="2" vid="V41001" fSeq="5" blockId="41"
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