Make Fn::GetAtt add a dependency

This will ensure that resources which get attributes from other resources
are not created until the latter resources are available.

An audit of all the templates in the heat-templates repo revealed that in
only one case did this cause a circular reference, and it was the template
that appeared to be incorrect (it has since been fixed).

Change-Id: Ibd51acdda55c24f665c5ca887040f3564d952c0e