Provide access to outputs from OS::Heat::ResourceGroup

To align with the related OS::Heat::AutoScalingGroup change:
Ib9e830e0c54602ac456d64179e1df30f0778906d

This adds an 'attributes' attribute to the OS::Heat::ResourceGroup resource
so that the following:
  {'get_attr': ['my_asg', 'attributes', 'networks', '<network_name>', 0]}
returns a mapping from resource names to the corresponding IP addresses.

There is no need for an outputs_list attribute since the following already
returns a list:
  {'get_attr': ['my_rg', 'networks', '<network_name>', 0]}

To align ResourceGroup with OS::Heat::AutoScalingGroup, there will
be a proposed change to OS::Heat::AutoScalingGroup to rename the
following attributes:
- outputs -> attributes
- outputs_list -> attributes_list

Change-Id: Id51f23533ae75ca5d883a4f67e75ac9513a67264
Merge "Provide access to outputs from OS::Heat::ResourceGroup"