identity: write db_sync logging in another log file

When bootstrapping cloud::identity, the DB is populated with
keystone-manage and tries to write logs in /var/log/keystone as root
user.
The problem is that if we don't use syslog, keystone won't be able to
start because of permissions issues, because Keystone can't write in
keystone.log who belongs to root user.
Using another configuration file ensure that:
* we can log the DB migration
* we won't write in keystone.log file and let Keystone start as
* expected.