[sql lab]Add autoSelect (#2713)

In sql editor database dropdown list, we want to auto-select the first available option.

superset/assets/javascripts/SqlLab/components/SqlEditorLeftBar.jsx

@@ -155,6 +155,7 @@ class SqlEditorLeftBar extends React.PureComponent {
             )}
             mutator={this.dbMutator.bind(this)}
             placeholder="Select a database"
+            autoSelect
           />
         </div>
         <div className="m-t-5">