Fixed ability to set field config from layout xml #11302 [backport 2.2]

[vovayatsyuk]

• This PR cherry-picked from MAGETWO-81310.
• Original PR.

Description

Config array is hardcoded to read three settings only. The patch allows reading all settings from config.

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>

Manual testing scenarios

1. Open Magento/Checkout/view/frontend/layout/checkout_index_index.xml and find postcode settings

2. Change its settings as follows:

   <item name="postcode" xsi:type="array">
       <!-- post-code field has custom UI component -->
       <item name="component" xsi:type="string">Magento_Ui/js/form/element/post-code</item>
       <item name="validation" xsi:type="array">
           <item name="required-entry" xsi:type="string">true</item>
       </item>
       <item name="config" xsi:type="array">
           <item name="label" xsi:type="string">test label</item>
           <item name="placeholder" xsi:type="string">test placeholder</item>
       </item>
   </item>

3. Navigate to checkout and you'll see that settings are not working.

4. Apply the patch and settings will start working.

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[dmanners]

@vovayatsyuk thanks for this. I will process this now for you.