CLAPPS: Add defaultImage logic (#4674)

* Add defaultImage logic

* Remove additional is-this-an-app? logic

src/features/linodes/LinodesCreate/LinodeCreateContainer.tsx

@@ -215,12 +215,11 @@ class LinodeCreateContainer extends React.PureComponent<CombinedProps, State> {
     defaultData?: any
   ) => {
     /**
-     * reset the selected Image but only if we're creating a Linode from
-     * a StackScript and not an app
+     * If we're switching from one cloud app to another,
+     * usually the only compatible image will be Debian 9. If this
+     * is the case, preselect that value.
      */
-    if (this.props.createType !== 'fromApp') {
-      this.setState({ selectedImageID: undefined });
-    }
+    const defaultImage = images.length === 1 ? images[0].id : undefined;
 
     this.setState({
       selectedStackScriptID: id,
@@ -230,7 +229,7 @@ class LinodeCreateContainer extends React.PureComponent<CombinedProps, State> {
       availableStackScriptImages: images,
       udfs: defaultData,
       /** reset image because stackscript might not be compatible with selected one */
-      selectedImageID: undefined,
+      selectedImageID: defaultImage,
       errors: undefined
     });
   };