[Forwardport] Fix adding values to system variable collection

[nmalevanec]

Original Pull Request

#13596
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)

[ihor-sviziev]

Hi @nmalevanec your changes looks good for me. Also I see that you covered these changes with unit test (that was missing in 2.2-develop branch). Could you create separate PR to 2.2-develop branch with adding test?

[magento-engcom-team]

Hi @ihor-sviziev, thank you for the review.
MAGETWO-88039 has been created to process this Pull Request

[nmalevanec]

@ihor-sviziev , done: #13742