Fixed ability to set field config from layout xml

Example:

```
<item name="postcode" xsi:type="array">
    <item name="config" xsi:type="array">
        <item name="label" xsi:type="string">Another Label</item>
        <item name="placeholder" xsi:type="string">Field Placeholder</item>
    </item>
    ...
</item>
```

Author: vovayatsyuk

app/code/Magento/Checkout/Block/Checkout/AttributeMerger.php

@@ -168,20 +168,18 @@ protected function getFieldConfig(
 
         $element = [
             'component' => isset($additionalConfig['component']) ? $additionalConfig['component'] : $uiComponent,
-            'config' => [
-                // customScope is used to group elements within a single form (e.g. they can be validated separately)
-                'customScope' => $dataScopePrefix,
-                'customEntry' => isset($additionalConfig['config']['customEntry'])
-                    ? $additionalConfig['config']['customEntry']
-                    : null,
-                'template' => 'ui/form/field',
-                'elementTmpl' => isset($additionalConfig['config']['elementTmpl'])
-                    ? $additionalConfig['config']['elementTmpl']
-                    : $elementTemplate,
-                'tooltip' => isset($additionalConfig['config']['tooltip'])
-                    ? $additionalConfig['config']['tooltip']
-                    : null
-            ],
+            'config' => $this->mergeConfigurationNode(
+                'config',
+                $additionalConfig,
+                [
+                    'config' => [
+                        // customScope is used to group elements within a single form (e.g. they can be validated separately)
+                        'customScope' => $dataScopePrefix,
+                        'template' => 'ui/form/field',
+                        'elementTmpl' => $elementTemplate,
+                    ],
+                ]
+            ),
             'dataScope' => $dataScopePrefix . '.' . $attributeCode,
             'label' => $attributeConfig['label'],
             'provider' => $providerName,