packaging: Don't change identity if no ENGINE_USER

Currently the parameters ENGINE_USER and ENGINE_GROUP in
/etc/sysconfig/ovirt-engine need to have a value. In order to simplify
use for developers this patch makes the value optional, so if they don't
have a value the service script will not change the identity before
starting the engine.

Change-Id: I0a56d73ad347011421ddd4f35671f3e39a610aec
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>