--mount-dest : dont modify mountpoint on target

Hello,

Thanks for the script, I am making a few modifications to fit my needs.

This pull requests adds the -a|--mount-dest flag which makes it so that the script does not set the mountpoint on target to none on replica creation.

This has to be used during replica creation else with the current logic of the script this will not work.

This is useful if you're serving the same dataset out of multiple servers as I am.