[ticket/11604] Use variables for config.php filesnames.

PHPBB3-11604
[ticket/11604] Fix case where config.php is not generated by phpBB.

PHPBB3-11604
[ticket/11604] Skip installer step where config.php is created.

PHPBB3-11604
Merge remote-tracking branch 'bantu/ticket/11604' into develop-olympus