lua/log: allow to use json formatter early

There is no reason to not allow for json formatter
on early logging stage.

We add verification that

	box.cfg{log="syslog:", log_format="json"}
or
	require('log').cfg{log="syslog:", format="json"}

is triggering error since syslog output requires
predefined structure and can't use json.

Fixes #5121

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>