fix ydump() for more complex yaml document.
add --volumes option to "mussel instance create"

For adding secondary blank volume:
  --volumes[0][size]=1G --volumes[0][volume_type]=local

For adding secondary volume from existing backup object:
  --volumes[0][backup_object_id]=bo-xxxxxx --volumes[0][volume_type]=local
add volume test cases for the instance with secondary volume.
stop using helper_instance for 1shot/t.instance.backup.
inherit volumes_args given out of setup_volumes() hook.