box: always reconfigure box at non-first box.cfg()

Calling box.cfg{} more than once does not normally cause any errors
(even though it might not have any effect). In contrast, assigning
it to some variable and then using it after the box was configured
caused an error since the method was overwritten by the initial call
of <load_cfg>.

The patch fixes this issue making box.cfg behave consistently in both
scenarios.

Follow-up #4231

Co-developed-by: Alexander Turenko <alexander.turenko@tarantool.org>