audit: rework configuration function

Let's obtain box.cfg parameters from audit_log_init with cfg_get instead
of passing them as arguments, like we do with the security module, for
example. This simplifies addition of new audit log parameters.

Needed for tarantool/tarantool-ee#502
Needed for tarantool/tarantool-ee#503

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring