Allow ResourceGroup to select an attribute path

Starting with the Juno version of HOT, ResourceGroup will take the path
into account when returning a list of attributes from get_attr. So
something like:

  {get_attr: [my_rsrc_group, networks, <network_name>, 0]}

will return a list of the first IP address of each server in the resource
group.

Closes-Bug: #1341048
Change-Id: I774dd14cd2e9ff878a214d5ea81faa6070bc48bf
Merge "Allow ResourceGroup to select an attribute path"