[ticket/11949] Do not prepend leading backslash to cache class name

The container seems to prepend the leading \ itself, so we get an
InvalidArgumentException with message '"'\\phpbb\\cache\\driver\\file'" is
not a valid class name for the "cache.driver" service.'

PHPBB3-11949
Merge pull request #1862 from nickvergessen/ticket/11949

[ticket/11949] Do not prepend leading backslash to cache class name