Resolving resource attributes fails on stack preview for some resources

[andersonvom]

Stack preview fails when previewing a stack with certain resources and throws this error:

ERROR: 'str' object has no attribute 'keys'
Traceback (most recent call last):

  File "/opt/heat/lib/python2.7/site-packages/oslo/messaging/rpc/dispatcher.py", line 137, in _dispatch_and_reply
    incoming.message))
  File "/opt/heat/lib/python2.7/site-packages/oslo/messaging/rpc/dispatcher.py", line 180, in _dispatch
    return self._do_dispatch(endpoint, method, ctxt, args)
  File "/opt/heat/lib/python2.7/site-packages/oslo/messaging/rpc/dispatcher.py", line 126, in _do_dispatch
    result = getattr(endpoint, method)(ctxt, **new_args)
  File "/opt/heat/lib/python2.7/site-packages/osprofiler/profiler.py", line 105, in wrapper
    return f(*args, **kwargs)
  File "/opt/heat/lib/python2.7/site-packages/heat/engine/service.py", line 72, in wrapped
    return func(self, ctx, *args, **kwargs)
  File "/opt/heat/lib/python2.7/site-packages/heat/engine/service.py", line 625, in preview_stack
    return api.format_stack_preview(stack)
  File "/opt/heat/lib/python2.7/site-packages/heat/engine/api.py", line 201, in format_stack_preview
    fmt_resources = map(format_resource, stack.preview_resources())
  File "/opt/heat/lib/python2.7/site-packages/heat/engine/api.py", line 198, in format_resource
    return format_stack_resource(res, with_props=True)
  File "/opt/heat/lib/python2.7/site-packages/heat/engine/api.py", line 185, in format_stack_resource
    resource, with_attr)
  File "/opt/heat/lib/python2.7/site-packages/heat/engine/api.py", line 137, in format_resource_attributes
    attributes = set(resolver.keys() + with_attr)

AttributeError: 'str' object has no attribute 'keys'

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/140199