lua/log: put string constants to map

This allows us to reuse them instead of copying
opencoded constants. They are highly bound to
ones compiled into the C code.

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>