Fix SoftwareDeployments validation

Fixes a regression caused by https://review.openstack.org/#/c/141444/

Instead of referencing the COUNT property, use _resource_names(),
which is already overridden in the SoftwareDeployments subclass
and is directly related to the count of created resources in
both resources.

Change-Id: Iea454f4201990f65633b644bbefe708e5a216885
Closes-Bug: #1404399
Merge "Fix SoftwareDeployments validation"