log: take into account env vars on log.cfg call

If env vars like TT_LOG, TT_LOG_LEVEL etc that specify log options are set
then log.cfg() should take them into account if called before box.cfg().

Closes #6011
Closes #7461

@TarantoolBot document
Title: log: take into account env vars on log.cfg call
Since: 2.11

If env vars like TT_LOG, TT_LOG_LEVEL etc that specify log options are set
then log.cfg() take them into account if called before box.cfg().