Add "index_var" property to the resource group resource

This property allows a template author to replace values in the
nested resource_ref with the index of the specific instance of
that resource in the group. This allows a user to, for example,
add the index of the resource to server names in order to make
them more easily distinguishable when listed outside of Heat.

Implemements: blueprint override-resource-name-in-resource-group
Change-Id: I4339067e3b8927573acff0ab5968de61a3253d07