Run 'keystone db_sync' as keystone user.

Running db_sync as root can cause permissions issues due to the fact
that the keystone log file gets owned as root
(/var/log/keystone/keystone.log).

This makes this module usable when using packages on Fedora 18.

Change-Id: I2c75db61474fb14f898c530bec2e8aad1230ad7c