Plugin Codeception - default path for reports.xml changed in v2.3

[phizzl]

Contribution Type: bug fix Link to Bug: https://github.com/Block8/PHPCI/issues/1331

This pull request affects the following areas:

• [ ] Front-End
• [ ] Builder
• [x] Build Plugins

In raising this pull request, I confirm the following (please check boxes):

• [x] I have read and understood the contributing guidelines?
• [x] I have checked that another pull request for this purpose does not exist.
• [x] I have considered, and confirmed that this submission will be valuable to others.
• [ ] I have created or updated the relevant documentation for this change on the PHPCI Wiki.
• [ ] Do the PHPCI tests pass?

Detailed description of change: Added the option "ouput" for the Codeception plugin. With the output option you may change the directory where to look for the report.xml. By default it will be set to _output/ since Codeception v2.3 stores it here.