Implement OS::Heat::StructuredDeployments

This is a resource which subclasses SoftwareDeployments to deploy a group
of OS::Heat::StructuredDeployment resources

Change-Id: I5f9603c68bb7cd81ef60a6b2ba965e5b8e93887a
Implements-Blueprint: deployment-multiple-servers
Merge "Implement OS::Heat::StructuredDeployments"