fix attach volume bug with resteasy

Fixed #111 :Attach volume now return a volumeAttachment, but it does not really make sense.We don't know whether this attach opreation will successd from the  volumeAttachment. And there is already CinderVolumeAttachment class which have same  structure existing. So I remove device ,id ,server from it,and change it a entity for attaching volume and solve attachving volume's bug with resteasy