fix zap command to require both hostname and disk, erroring properly

Improves the wording of the error (before we were getting an ugly pushy traceback) and fixes the help menu to reflect what the zap function needs to function properly

Example of error handling:

    ceph-deploy disk zap node2
    [ceph_deploy][ERROR ] RuntimeError: zap command needs both HOSTNAME and DISK but got "node2 None"