identity: write db_sync logging with keystone user

When bootstrapping cloud::identity, the DB is populated with
keystone-manage and tries to write logs in /var/log/keystone as root
user by default.
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.

We should use 'keystone' user to run the command.
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>