7241 Revert commit eb79b38b and re-add constants

See https://github.com/magento/magento2/pull/11462#issuecomment-348441645

app/code/Magento/Config/Model/Config/Source/Nooptreq.php

@@ -11,15 +11,19 @@ namespace Magento\Config\Model\Config\Source;
  */
 class Nooptreq implements \Magento\Framework\Option\ArrayInterface
 {
+    const VALUE_NO = '';
+    const VALUE_OPTIONAL = 'opt';
+    const VALUE_REQUIRED = 'req';
+
     /**
      * @return array
      */
     public function toOptionArray()
     {
         return [
-            ['value' => '', 'label' => __('No')],
-            ['value' => 'opt', 'label' => __('Optional')],
-            ['value' => 'req', 'label' => __('Required')]
+            ['value' => self::VALUE_NO, 'label' => __('No')],
+            ['value' => self::VALUE_OPTIONAL, 'label' => __('Optional')],
+            ['value' => self::VALUE_REQUIRED, 'label' => __('Required')]
         ];
     }
 }

app/code/Magento/Customer/Model/Options.php

@@ -45,7 +45,7 @@ class Options
     {
         return $this->prepareNamePrefixSuffixOptions(
             $this->addressHelper->getConfig('prefix_options', $store),
-            $this->addressHelper->getConfig('prefix_show', $store) == 'opt'
+            $this->addressHelper->getConfig('prefix_show', $store) == NooptreqSource::VALUE_OPTIONAL
         );
     }
 
@@ -59,7 +59,7 @@ class Options
     {
         return $this->prepareNamePrefixSuffixOptions(
             $this->addressHelper->getConfig('suffix_options', $store),
-            $this->addressHelper->getConfig('suffix_show', $store) == 'opt'
+            $this->addressHelper->getConfig('suffix_show', $store) == NooptreqSource::VALUE_OPTIONAL
         );
     }