Fix adding values to system variable collection

[mszydlo]

Changed select fields in joined variable value table in Magento\Variable\Model\ResourceModel\Variable\Collection#addValuesToResult() method to match DB schema

Description

Method mentioned above tried to select field value from joined variable_value table (which doesn't exist). I've replaced incorrect field with two proper ones: plain_value and html_value.

Fixed Issues

Calling Magento\Variable\Model\ResourceModel\Variable\Collection#addValuesToResult() method results with Zend_Db_Statement_Exception thrown. Exception message:

SQLSTATE[42S22]: Column not found: 1054 Unknown column 'value_table.value' in 'field list', query was: SELECT `main_table`.*, `value_table`.`value` FROM `variable` AS `main_table`  
   INNER JOIN `variable_value` AS `value_table` ON value_table.variable_id = main_table.variable_id WHERE (`value_table`.`store_id` = 0)

Manual testing scenarios

As the method is not used anywhere in the magento core there is no way to reproduce error with just clicking.

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]

@mszydlo thank you for this PR. I will process this now and get back to you if I need anything else.

[magento-engcom-team]

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