Gracefully finish drive operations on demand and put the drive down

New feature of cta-taped is needed in order to gracefully request to finish the current batch of files and stop = put the drive down. Trigger this action based on some pattern match: library=XXX or VO=XXX or drive=XXX or tapeserver=XXX

This is also related to this cta-taped milestone: %Tape Daemon Roadmap