Fix some error handling changes
Some changes to error handling were introduced to try and properly handle the case where failed JSON resulted in an err being returned as expected. But the data structures were left possible partially populated.
The previous branch returned nil for this situation but non pointers cannot be set to nil.
This branch use pointers for return data types of nova and glance apis so that nil can be returned if there is an error.
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https://codereview.appspot.com/6869044