REST API call to fetch an arbitrary resource attribute

Currently the attributes of a given resource are not known unless they are included in stack outputs, and the stack is in a COMPLETE state.

Having a REST API call to fetch the value of a resource attribute would help in the following circumstances:
* Investigating stack state while still IN_PROGRESS
* debugging problem stacks
* discovering attribute values without the forsight of adding a stack output to the template

heatclient can implement the following command

heat resource-attr <stack_id> <resource_name> <attr_name>