fix[DragTable]: support multiple drag-table (#1666)

src/views/table/dragTable.vue

 <template>
   <div class="app-container">
     <!-- Note that row-key is necessary to get a correct row order. -->
-    <el-table v-loading="listLoading" :data="list" row-key="id" border fit highlight-current-row style="width: 100%">
+    <el-table v-loading="listLoading" ref="dragTable" :data="list" row-key="id" border fit highlight-current-row style="width: 100%">
 
       <el-table-column align="center" label="ID" width="65">
         <template slot-scope="scope">
@@ -107,7 +107,7 @@ export default {
       })
     },
     setSort() {
-      const el = document.querySelectorAll('.el-table__body-wrapper > table > tbody')[0]
+      const el = this.$refs.dragTable.$el.querySelectorAll('.el-table__body-wrapper > table > tbody')[0]
       this.sortable = Sortable.create(el, {
         ghostClass: 'sortable-ghost', // Class name for the drop placeholder,
         setData: function(dataTransfer) {