Added file_name parameter for fixtures.yml

Bug fix: [no]
Feature addition: [yes]
Backwards compatibility break: [yes]
Unit test passes: [yes]
Specs passes: [yes]
Replaced unit test classes removed: [no]
Behat scenario passes: [yes]
Checkstyle issues: [no]*
ChangeLog updated: [no]
Documentation PR: [link to the documentation PR if there is one]
Fixes the following jira:

This PR adds a new 'file_name' property for fixtures.yml configuration files. This basically allows to have multiple fixtures readers reading the same file, which is required for the xslx single file initialisation.