Rename generic variable named with mysql specific name

Method migrate_data in VolumeDevice class is called by all datastores.
Input variable "mysql_base" and method's description are  misleading.

Change-Id: I5c71018f8ce61b9673dd83f4fa790c8cd0dfc954
Closes-Bug: #1393511
Updated from global requirements

Change-Id: I800b86b2ce700752422a80ccf53d2bc6c63095ea
Merge "Rename generic variable named with mysql specific name"