core/say: do not reconfig early set up logger

We gonna support logger configuration on its own
without requirement to call `box.cfg{}`. Thus lets
say_logger_init() to skip processing if we already
did.

Note it is preparatory for next patches. Currently
the init called once.

Part-of #689

Reviewed-by: Oleg Babin <olegrok@tarantool.org>
Reviewed-by: Leonid Vasiliev <lvasiliev@tarantool.org>
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>