In its current form, cinder::backend::iscsi is a define, allowing to call
it multiple times, which is desirable (if a server has multiple backends).
However, some resources are defined twice if the class is called twice.
To avoid this, this patch decorates the resource definition for packages
and services with if !defined.
Change-Id: I779dfd416da6fe8bb713105ed246172ebf0aae85