config: create parent directory for log file

For example, if we want the logs to be in a separate `var/log`
directory, it is convenient to just configure it as `var/run/{{
instance_name }}.log` and let tarantool create the parent directory.

Part of #8862

NO_DOC=This change is too minor to describe in the documentation issue
       https://github.com/tarantool/doc/issues/3544. I'll work with the
       documentation team regarding details of startup/reload flow and
       we'll determine what should go to the user documentation and what
       shouldn't.