Add a server with volumes ResourceGroup example

Adds an example in response to a ML question, shows how you can
create a resource group containing a number of nested stacks containing
identical volumes and volume attachment resources, then pass in the
server ID to have the volumes attached to one server.

Also moves the current basic example into a resource_group subdirectory,
so we can collect related examples, and make more obvious that the
volume_with_attachment template is part of another example rather than
a standalone example template.

Change-Id: I1ffc1656286a981b6dc219038e03adbb8d5af775
Merge "Add a server with volumes ResourceGroup example"