ResourceGroup build from list of resource names

This changes ResourceGroup internal implementation to be driven by
a list of resource names rather than a count of resources.

This maintains the current behaviour of sequential number resource
names, while allowing subclasses to choose a completely different
resource name scheme.

Change-Id: I17d046155f8d830ced789ede6a969ac4f125576d
Partial-Blueprint: deployment-multiple-servers
Merge "ResourceGroup build from list of resource names"